I’m working on a project that need some complicated html snippets for test, which cannot be easily generated with factory. So I put these snippets into fixture files.

RSpec provides a very convenient DSL keyword let, which allow us to define something for test and cached it in the same test. And I want I could have some similar keyword for my html fixtures. To achieve this goal I decide to extend DSL.

So I created module which contains the new DSL I want to have:

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# spec/support/html_pages.rb
module HtmlPages
  def load_html(name)
    let name do
      file = Rails.root.join('spec/html_pages', "#{name}.html")
      Nokogiri::Html File.read(file)
    end
  end
end

Put this file into the path spec/support, by default, spec_helper.rb would require this file for you.
Then we should tell rspec to load the DSL into test cases.

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RSpec.configure do |config|
	# ...
	config.extend HtmlPages
	# ...
end

By telling config to extend the module, our DSL will be loaded as the class methods of RSpec::Core::ExampleGroup, where let is being defined.

HINT: Rspec config has another way to extend DSL by calling config.include. Then the DSL methods will be injected into the test example group instance, then these methods can be used in the test cases. That’s how runtime DSLs like FactoryGirl work.

Just met a super wield question in zsh when I trying to invoke a parameterized rake task.
I have a rake task that helps me to create class and related test case for coffee script. It should be invoked in this way:

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rake new:class[class_name]

I need to pass the parameter in a pair of square brackets, the syntax work pretty fine in bash shell. But when I do it in zsh, it yield error:
zsh: no matches found: new:class[BottleContainer]

So I have to create a new bash instance in zsh to invoke the shell. It is super wield and stupid.

Now I’m hacking some web sites to try to grab the interesting informations from the page. But for some reason, all the sites I’m working on now, doesn’t included jQuery, they uses YUI or extJS, which makes me feel super inconvenient when trying hacking the page pattern.

Do I need to find a way to inject jQuery into the current page that I’m browsing.
Thanks to the browser address bar which allow us to execute javascript directly on current session. And also thanks to bookmark, which allow us to persists the script into bookmark, and invoke it with only one mouse click.

So I wrote this:

With this, we can quickly inject jQuery into current session in no time.

You can drag the following link to your browser favorites bar to:
Inject jQuery

Postgres provided a very interesting feature called “Schema” in addition to other “normal” database features, which provide a extra layer between database and tables. So with schema, you can have tables with same name in one database, if they are in different schemas.
To me schema is not a good idea! I assume “table-space” or even “namespace” could be a better name. In fact, there are a number of people agree that schema is not a good name:

“Schema” is such a terrible name for this feature. When most people hear the term “schema” they think of a data definition of some sort. This is not what PostgreSQL schemas are. I’m sure the PostgreSQL devs had their reasons, but I really wish they would have named it more appropriately. “Namespaces” would have been apropos.

Anywho, the easiest way for me to describe PostgreSQL schemas (besides telling you that they are, indeed, namespaces for tables) is to relate them to the UNIX execution path. When you run a UNIX command without specifying its absolute path, your shell will work its way down the $PATH until it finds an executable of the same name.

And there is a popular routine is to use the postgres schema for sub-domains. For example, you’re a BSS provider, you rend your BBS apps to different organizations. To the organizations. they want to have its own BBS app instance running independently, the most important is that data should be stored into separated spaces, and could be accessed from its own domain name. But to you, for administration, you want they share the same backend management console.
In this case the best way to solve the problem is to store the data owned by different subsystem into different schemas. But store all the administration data into a single schema or even in public schema.
The same guy Jerod has a post described how to build this kind of system in details. There are a bunch of posts described how to build the system like this, which could be found by googling easily.
And there is even a ruby gem called apartment from Brad Robertson to support this kind of system

This idea looks fancy, but unless you 10000% certain that the sub-systems will keep its independent status and without any collaboration forever.
Or it sooner or later, you will find the “fancy idea” become a horrible idea.
When time goes by, there could be more and more and more features that required to add collaboration between sub-systems. Such as provide a unified authentication mechanism, so user can logged in once and switch between different systems easily. Or administrator might ask for a unified statistics graph for all sub-systems.
All these requirements are related to cross-schema query! To be honest, cross query in some cases could be painful!
And it brings trouble to all aspects in your system, such as data migration, test data generation, etc.

That’s what exactly happens in my current project!
My current project is Rails 3 project, the codebase is brand new but built on a legacy multiple schema postgres database. And for some reason,
we must keep the multiple schema design unchanged. But our goal is to unify the separated subsystem into a more closed-collaborated system.

Since ActiveRecord in Rails doesn’t include the native support to this fancy feature. Which means you will met problem during migration, or even preparing test data with factory-girl.

Postgres allows to locate the table in different schemas with full qualified name like this <schema name>.<table name>.<column name>. The schema name is optional, when you omitted the schema name, Postgres will search the table in a file-system-path-like order called “search-path“.

And you can set and query current search path with Postgres SQL statements:

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SHOW search_path;
SET search_path TO <new_search_path>;

Since ActiveRecord won’t add the full qualified schema name in front of the table name when it translate the ARel into SQL statements. So we can only support the multiple schema database with the search_path.

Basically, it is a very natural idea that you can use the following ruby code to make ActiveRecord make query on different schemas:

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def add_schema_to_search_path(schema)
  ActiveRecord::Base.connection.execute "SET search_path TO #{schema}, public;"
end
def restore_search_path
  ActiveRecord::Base.connection.execute "SET search_path TO public;"
end

This two methods work perfect when querying things from the database. But sooner or later, you will run into big trouble when you try to write data into database.

In db migration or use factory_girl to generate test fixtures, you might found that the data you insert in different schemas finally goes into the first non-public schema. But all the query still works perfect!

We found this problem occurs when the following conditions are satisfied:

1. Query are happened in a Transaction.
2. You insert data into multiple non-public schemas.
3. You user SET search_path TO SQL statement to switch between schema rather than explicitly using full-qualified table name.

And the most interesting thing is that:

1. All the SELECT queries are executed on schemas correctly
2. If you use SHOW search_path; to query current search path, you will got correct search path value.
3. All data are inserted into first non-public schema that you actually wrote data into. So which means it you try to insert data into public schema, it won’t go wrong. Or you switched to a non-public schema, but actually you doesn’t insert any rows, it also won’t be impacted.

To solve this problem, I spent 2 nights and 2 days to digged into the source code of ActiveRecord gem and pg gem (the Postgres database adapter).
And finally I solved the problem by using the attribute on PostgreSQLAdapter.

Basically, instead of using the SQL query, you should use the PostgreSQLAdapter#schema_search_path and PostgreSQLAdapter#schema_search_path= to get and set the search path.
And if you dig into the source code, you will find the two methods does the exact same thing as we did except it assigned one more instance variable @schema_search_path.

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# Sets the schema search path to a string of comma-separated schema names.
# Names beginning with $ have to be quoted (e.g. $user => '$user').
# See: http://www.postgresql.org/docs/current/static/ddl-schemas.html
#
# This should be not be called manually but set in database.yml.
def schema_search_path=(schema_csv)
  if schema_csv
    execute("SET search_path TO #{schema_csv}", 'SCHEMA')
    @schema_search_path = schema_csv
  end
end
# Returns the active schema search path.
def schema_search_path
  @schema_search_path ||= query('SHOW search_path', 'SCHEMA')[0][0]
end

The most interesting thing is if you search the reference to @schema_search_path, you will find it is only used as a local cache of current search_path in the adapter, and it is initialized with the value from the query SHOW search_path; if it is nil, and then keep the value as the cache!
This implementation is buggy and caused the problems described before!

If we use the SQL query to set the search path rather than calling schema_search_path=, we won’t set the @schema_search_path at sametime, ideally this value will remain nil by default. Then transaction or other object in ActiveRecord call schema_search_path to get current search path. The first time, the variable @@schema_search_path is nil, and will be initialized by the value from query SHOW search_path; and then won’t changed any more, since in the future this query won’t be executed any longer.
As a result, the schema will be switched successfully for the first time, but failed in the following.

Which means at current stage, if you want to change search_path, the only correct way is to use PostgreSQLAdapter#schema_search_path=, and PLEASE PLEASE ignore the warning "This should be not be called manually but set in database.yml." in the source code! It is really a misleading message!

I understand current implementation is for performance consideration, but caching the value is absolutely not a good idea when you cannot keep things in sync and the sync is critical in some cases.
I’m planning to fix this issue in rails codebase and create a pull request to rails maintainer. Wish they could accept this fix. Or at least they should change the warning message.

And besides of using the out-dated and mysterious PgTools mentioned in a lot of posts (I saw a lot of people mentioned this class, but I cannot find it anywhere even I from google or github. It is really a mystery). I create a new utility module called MultiSchema.

You can use it as the utility class in the old-fashioned way:

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MultiSchema.with_in_schemas :except => :public do
  # Play around the data in one schema
end

Or you can use it in a DSL-like way:

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class SomeMigration < ActiveRecord::Migration
include MultiSchema
  def change
    with_in_schemas :except => :public do
      # Play around the data in one schema
    end
  end
end

with_in_schemas method accept both symbol and string, and you can pass single value, array or hash to it.

• with_in_schemas yield all user schemas in the database
• with_in_schemas :only => %w(schema1 schema2) populates all given schemas.
• with_in_schemas :except => %w(schema1 schema2) populates all except given schemas.
• with_in_schemas :except => [:public] is equivalent to with_in_schemas :except => ['public']
• with_in_schemas :only => [:public] is equivalent to with_in_schemas :only => :public and equivalent to with_in_schemas :public
• with_in_schemas :except => [:public] is equivalent to with_in_schemas :except => :public

In dynamic language, most language doesn’t provide the function overload mechanism like static language does, which means you cannot define functions with same name but different arguments, and the language itself won’t help you to dispatch the call according to the arguments.
So you have to deal with the overload by yourself in dynamic languages.

In dynamic language, since you have to dispatch the call by yourself, so you can play some tricks during process the arguments. The most common trick is grammar sugar, which can help you simplify the code and increase the readability. And it is a very important feature when you’re building DSL.

Syntax sugar in argument means you can omit something unnecessary or unimportant parameters in specific context, then the function will try to infer and complete the parameters. For example, developer should be able to omit the password parameter if the system enable authentication mechanism. These kind of tricks are very common in dynamic language frameworks, such as jQuery.

Here is a list of some common grammar sugar cases:

• Set default value for omit parameter, such as for jQuery animation function jQuery.fadeIn([duration] [, callback] ), you can omit the parameter duration, which is equivalent to provide duration as 400
• Provide different type of value for a same parameter, still the jQuery animation function jQuery.fadeIn([duration] [, callback] ), you can provide number for duration, or you can provide string “fast” and “slow” as the value of duration.
• Provide a single value rather than a complex hash, such as function jQuery ajax function jQuery.ajax(settings), you can provide a url string, which is equivalent to provide a hash{url: <url string>}
• Pass a single element instead of a array of the element.

After we analyze the cases, we will find that all the grammar sugar is to allow user to provide exactly same information but in different formats. Since all the data are same piece of information, so it should be possible to unify all the information into one format! Which means to support grammar sugar, the major problem is to unify the type of parameter.
Besides unify the type, another important problem is to handle the null values, such asnull and undefined in javascript, nil in ruby, etc.

Here is a piece of ruby code that I parse the argument by unifying the parameter type:

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  def apply_to_items(options = nil)
    options = unify_type(options, Hash) { |items| {:only => items} }
    options[:only] = unify_type(options[:only], Array) { |item| item.nil? ? list_all_items : [item]  }
    options[:except] = unify_type(options[:except], Array) { |item| item.nil? ? [] : [item] }
    options[:only] = unify_array_item_type(options[:only], String) { |symbol| symbol.to_s }
    options[:except] = unify_array_item_type(options[:except], String) { |symbol| symbol.to_s }
    target_items = options[:only].select { |item| options[:except].exclude? item }
    target_items.each do |item|
      yield item
    end
  end
  private
  def list_all_items
	# return all items fetched from database or API
	# ...
  end
  def unify_type(input, type)
    if input.is_a?(type)
      input
    else
      yield input
    end
  end
end

And here is the test for the code:

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require 'spec_helper'
describe Module  do
  before do
    extend Moudle
  end
  it "should populate all items" do
    visited = []
    apply_to_items do |item|
      visited << item
    end
    visited.should =~ %w(public another_item)
  end
  describe "should populate the provided items" do
    it "provide as string array" do
      visited = []
      apply_to_items(%w(another_item)) do |item|
        visited << item
      end
      visited.should =~ %w(another_item)
    end
    it "provide as symbol array" do
      visited = []
      apply_to_items([:another_item]) do |item|
        visited << item
      end
      visited.should =~ %w(another_item)
    end
    it "provide as string item" do
      visited = []
      apply_to_items('another_item') do |item|
        visited << item
      end
      visited.should =~ %w(another_item)
    end
    it "provide as symbol item" do
      visited = []
      apply_to_items(:another_item) do |item|
        visited << item
      end
      visited.should =~ %w(another_item)
    end
    it "provide as string array in hash" do
      visited = []
      apply_to_items(:only => %w(another_item)) do |item|
        visited << item
      end
      visited.should =~ %w(another_item)
    end
    it "provide as symbol array in hash" do
      visited = []
      apply_to_items(:only => [:another_item]) do |item|
        visited << item
      end
      visited.should =~ %w(another_item)
    end
    it "provide as string item in hash" do
      visited = []
      apply_to_items(:only => 'public') do |item|
        visited << item
      end
      visited.should =~ %w(public)
    end
    it "provide as symbol item in hash" do
      visited = []
      apply_to_items(:only => :public) do |item|
        visited << item
      end
      visited.should =~ %w(public)
    end
  end
  describe "should except the not used items" do
    it "except as string item in hash" do
      visited = []
      apply_to_items(:except => 'public') do |item|
        visited << item
      end
      visited.should =~ %w(another_item)
    end
    it "except as symbol item in hash" do
      visited = []
      apply_to_items(:except => :public) do |item|
        visited << item
      end
      visited.should =~ %w(another_item)
    end
    it "except as string array in hash" do
      visited = []
      apply_to_items(:except => %w(public)) do |item|
        visited << item
      end
      visited.should =~ %w(another_item)
    end
    it "except as symbol array in hash" do
      visited = []
      apply_to_items(:except => :public) do |item|
        visited << item
      end
      visited.should =~ %w(another_item)
    end
  end
end

The algorithm in previous code is language independent, so ideally, it could be reused in any language, such as java script or python.

Comparison to NULL

In postgres, NULL is treat as a speical value, that which is not equal to any other value, which means the expression NULL = NULL yields false.
It can be verified by using the following query

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SELECT n
FROM unnest(ARRAY(NULL,1,2,3,4,5)) n
WHERE n = NULL

The query returns empty set, because no element equals to NULL even NULL itself.
If you think this experiment is not convincing enough, then you can try this:

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SELECT
	n,
	CASE WHEN n = NULL THEN 'NULL' ELSE 'NOT NULL' END
FROM unnest(ARRAY(NULL,1,2,3,4,5)) n

This query should yield “ , NOT NULL” which means NULL does not equal to NULL.

To test whether or whether not a value equals to NULL, you should use IS NULL or IS NOT NULL

So if you replace the n = NULL with n IS NULL in previous 2 statements, you will get expected result:

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SELECT
	n,
	CASE WHEN n IS NULL THEN 'NULL' ELSE 'NOT NULL' END
FROM unnest(ARRAY(NULL,1,2,3,4,5)) n
WHERE n IS NULL

NULL in Crosstab

In most cases, the NULL special character doesn’t hurt much, since we always can alter our expression to fix the problem. But if you’re using table functions to create pivot, and there is NULL in your columns, then you will find NULL is a cursed value, which brought a lot of trouble to you.

Postgres provide tablefunc extension, which can provide a series functions called “crosstab#”. And with these functions, you can convert a set of rows into a pivot table.

Function crosstab accept 2 sql queries. First query should yield 3 columns: row in pivot table, column in pivot table and value in pivot table.
Second query should yield a series of value which defines the columns of the pivot table.
crosstab function will group the rows yield by 1st query by 1st column. Then map each row in the group to a column by comparing the 2nd column in the row yield by 1st query with the value generated by 2nd query, if the value is equal then the 3rd column of the row yield by 1st query will be placed in the column defined by value of the 2nd query.
It if a very convenient feature provided by postgres, and it works perfect in most cases.

But in our case, we met a problem that we have NULL value in the 2nd column of 1st query, which means we have NULL value in pivot table columns!

And the crosstab decide which column the value should be placed to by comparing the whether the value is equal!

And NULL never equals to NULL!

BAM!!!

As a result, the column of NULL in the pivot table is always empty!

CASE WHEN

To solve the problem, we should play a little trick in the first query, we should translate all the NULL into a “normal” value.
Here is our 1st query, and we want to get a pivot table with period as row axis, rating as column axis and volume of the order as content.

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SELECT
	Periods.period,
	Profiles.rating,
	SUM(Orders.volume)
FROM
	orders
		LEFT OUTTER JOIN Periods ON (Periods.id = Orders.period_id)
		LEFT OUTTER JOIN Profiles ON (Profiles.id = Orders.profile_id)
GROUP BY
	Periods.period,
	Profiles.rating
ORDER BY
	Periods.period,
	Profiles.rating

And we will get NULL in Profiles.rating, so we can translate NULL as 0 in rating. To achieve this we can use CASE WHEN statement.

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SELECT
	Periods.period,
	CASE WHEN Profiles.rating IS NULL THEN 0 ELSE Profiles.rating,
	SUM(Orders.volume)
FROM
	orders
		LEFT OUTTER JOIN Periods ON (Periods.id = Orders.period_id)
		LEFT OUTTER JOIN Profiles ON (Profiles.id = Orders.profile_id)
GROUP BY
	Periods.period,
	Profiles.rating
ORDER BY
	Periods.period,
	Profiles.rating

COALESCE

The solution works fine. But in personal perspective, I don’t like it, because it repeat the statement and is not concise. But luckily, the value we should deal with the the special value NULL and postgres has provided a group of functions to deal with NULL.

What we want is the function COALESCE, which accept a group of value as arguments, and returns the first not null value.
So we can simplify our statement with this super function:

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SELECT
	Periods.period,
	COALESCE(Profiles.rating, 0)
	SUM(Orders.volume)
FROM
	orders
		LEFT OUTTER JOIN Periods ON (Periods.id = Orders.period_id)
		LEFT OUTTER JOIN Profiles ON (Profiles.id = Orders.profile_id)
GROUP BY
	Periods.period,
	Profiles.rating
ORDER BY
	Periods.period,
	Profiles.rating

In the statement, if the rating is not null, the COALESCE function will return the actual value if the rating is NULL, then the COALESCE will find the next not null value, which must be 0.

Besides COALESCE function, there is another function called NULLIF, which might mislead you to a totally wrong way just as what I had.
According to postgres document, the function might behave in a totally opposite way than you expected.

The NULLIF function returns a null value if value1 equals value2; otherwise it returns value1. This can be used to perform the inverse operation of the COALESCE

Today, I was so surprised that I got an empty string when I call the serialize method on a jQuery wrapped form.
The html is written in Haml:

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%form#graph-option.horizontal-form
	%fieldset
		%label{ :for=>'start-date'} Start Date
		%select#start-date
		%label{ :for=>'end-date'} End Date
		%select#end-date
		%button#submit-option.btn.large-btn

And the script is written in coffee-script:

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$ ->
	$('#submit-option').click ->
		option = $('#graph-option').serialize()
		$.post '/dashboard/graph', option, (data) ->
			renderCharts data

When I execute the script, I got 500 error. And the reason is that the option is empty.
I believe this must be caused by a super silly mistake, so I try to call serialize methods on Twitter Bootstrap website, and I still got empty string!!!!

After half an hour debugging, I just realize that I forgot to assign the name to all the input elements. And according to html specification, the browser uses the name of the elements to identify whom the value belongs to.
So when the name is omitted, the serailizeArray method in jQuery returns an empty array, as a result, the serialize method returns empty string.

According to my experience, it is easy to identify this problem, if the html is in html-like format, such as erb. But it is really hard to identify this issue if the page is written in haml, because in haml, id is used much more often.
To fix this problem, we need to specify the name explicitly for each form element.

Here is the fixed haml code:

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%form#graph-option.horizontal-form
	%fieldset
		%label{ :for=>'start-date'} Start Date
		%select#start-date{ :name=>'start-date' }
		%label{ :for=>'end-date'} End Date
		%select#end-date{ :name=>'end-date' }
		%button#submit-option.btn.large-btn

I’m a newbie to Rails, and my past few projects are all rails based, including MetaPaas, Recruiting On Rails and current SFP.
I was amazed by the convenience of Rails, and also hurt by its “smartness”.
The power of Rails was described in quite a lot of posts, so I wanna to share some failure experiences.
Actually I have felt into quite a number of pitfalls in Rails, and here is one of the most painful ones.

To explain the problem easier, I just simplify the scenario:
I have a model called “Candidate”, which holds a “Status” to store the status of the candidate, so I have the code like this:

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class Candidate < ActiveRecord::Base
	has_one status
	# other definition
end
class Status < ActiveRecord::Base
	belongs_to candidate
	# other definition
end

For some reason, I change the relationship between Candidate and Status. It is changed from one-to-one to one-to-many.
So I changed the has_one to has_many:

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class Candidate < ActiveRecord::Base
	has_many status
	# other definition
end
class Status < ActiveRecord::Base
	belongs_to candidate
	# other definition
end

I thought it is an easy modification, but the app fails to run, even I have done the database migration.
It said rails cannot find a constant named “Statu”!

After my first sight on this error message, I believed it is caused by a typo, I must mistyped “Status” as “Statu”.
So I full-text search the whole project for “Statu”, but I cannot find any.

This error message is quite weird to me, since I have no idea about where the word “Statu” come from!
After spent half an hour on pointless trying, I suddenly noticed that the word “status” is end with “s”, and according to Rails’ convention, rails must think “status” is the plural form of “statu”. So according to the convention again, it try to find a class named “Statu”.
And we should use the plural form noun as name for one-to-many field, since that holds an array rather than a single object.

So after changing status to statuses, the problem solved.

Convention based system is powerful, a lot magic just happened there. But also the magic things are hard to debug when some special case breaks the convention presumption.

HomeBrew is a convenient package manager for Mac user. For some reason I prefer Home Brew to Mac Ports.
Brew has a younger package repository since it has shorter history comparing to MacPorts. Younger repository means less options. And sometime it is hard for you to install the old-fashioned tool with brew.

Brew uses git to manage its formula repository, so you can list with git.
Typically, the repo is located at /usr/local. But since this path can be changed, so it is safer to reference this path via brew.
Brew call the repo path as prefix, so you can reference the path with brew --prefix
You can use the following shell command to enter the brew repo.

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cd $(brew --prefix)

Since brew load formula from local, so before we install the app with brew, we need to ensure the repo is updated. We can use the following command to update the brew repo:

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# Update brew
brew update
# update with git
cd $(brew --prefix) && git pull --rebase

To install specific version of the app, we need to checkout the specific version of the formula, we can get the versions and related git revision by brew versions command, and checkout specific version, then install the app:

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brew versions postgresql
# Output:
# 9.1.3    git checkout e088818 /usr/local/Library/Formula/postgresql.rb
# 9.1.2    git checkout dfcc838 /usr/local/Library/Formula/postgresql.rb
# 9.1.1    git checkout 4ef8fb0 /usr/local/Library/Formula/postgresql.rb
# 9.0.4    git checkout 2accac4 /usr/local/Library/Formula/postgresql.rb
# 9.0.3    git checkout b782d9d /usr/local/Library/Formula/postgresql.rb
# 9.0.2    git checkout 2c3b88a /usr/local/Library/Formula/postgresql.rb
# 9.0.1    git checkout b7fab6c /usr/local/Library/Formula/postgresql.rb
# 9.0.0    git checkout 1168d8f /usr/local/Library/Formula/postgresql.rb
# 8.4.4    git checkout c32bea0 /usr/local/Library/Formula/postgresql.rb
# 8.4.3    git checkout 9b2ef7c /usr/local/Library/Formula/postgresql.rb
# 8.4.1    git checkout 0495cf5 /usr/local/Library/Formula/postgresql.rb
# 8.4.0    git checkout a82e823 /usr/local/Library/Formula/postgresql.rb
git checkout a82e823 /usr/local/Library/Formula/postgresql.rb
brew install postgresql

If we cannot find the specific version that we want (Such as Postgres 8.3.11). don’t be disappointed, we can try to search the version repository.
Some of the old-fashioned tool which is not included in brew’s master repo might be provided in version repository.

Begin from Brew 0.9 provide the multiple repository support, user can use brew tap command to register alternative repositories besides the master repo. There are quite a some interesting alternative repos, such as versions and games.
These official alternative repos can be found on github

The formulas in alternative repositories cannot be used directly, but luckily the official ones are included in the search result.

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brew search postgresql
# Output:
# postgresql
# homebrew/versions/postgresql8    homebrew/versions/postgresql9

In the search result, we can see there are 2 formulas are displayed with a path rather than just the formula name, which means these formulas are in a alternative repo.
The path to the formula follows the convention: <github username>/<repository name without "homebrew-">/<formula name>.
So homebrew/versions/postgresql8 means the file is located at https://raw.github.com/Homebrew/homebrew-versions/master/postgresql8.rb

To install it, we can install it directly or tap the repo first:

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# Install directly
brew install homebrew/versions/postgresql8
# Tap
brew tap homebrew/versions
brew install postgres8

Coffee Script borrowed quite a lot syntax patterns from both Ruby and Python, especially from Ruby.
So people, like me, usually tends to write coffee-script in ruby way.

In ruby, we can retrieve the element in an array in reversed order by using a negative index, which means array[-1] returns the last element in the array. This grammar sugar is really convenient and powerful, so we can omit the code like this array[array.length - 1].

But for some reason, coffee-script doesn’t inherit this syntax. To me, it is weird. But after analyze this problem in detail, I found the reason.
Coffee script announce it has a golden rule: “coffee-script is just javascript”. So all the coffee script must be able to compiled into javascript.

Let’s try to analyze how the coffee script is compiled into javascript:

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array = [1..10]
second = array[1]
last = array[-1] // Psudocode

Obviously, the previous code should be compiled as following:

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var array, last, second;
array = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10];
second = array[1];
last = array[array.length - 1];

The negative index should be processed specially, so we should check the index is negative or not while compiling. This translation seems easy but actually not, since we can and usually use variable as the index.

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array = [1..10]
index = 1
second = array[index]
index = -index
last = array[index]

In the previous code, because we use the variable as index, which cannot be verified in compile-time, which means we need to compile the array reference code as following:

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var array, index, last, second;
array = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10];
index = 1;
second = index >=0 ? array[index] : array[array.length + index];
index = -index;
last = index >=0 ? array[index] : array[array.length + index];

So every time we reference the array, we need to check whether the index is negative or not. This approach absolutely hurts the performance a lot, which in basically unacceptable.
So that’s why coffee-script doesn’t support the negative index.