Rails multiple databases

May 27, 2009 at 12:02 pm (Model) (, , , , )

Consider the case where some tables exist in database A and the others in B. Depending on the table under question, we have to connect to the specific Database and query it.

Supposing you have two databases say A,B.
Now you need configurations for both of them in database.yml (AS each RAILS configuration can point only one DATABASE)
let the development environment point database “A”
So create another environment called “dummy” point at database “B”

development:
adapter  : mysql
username : user-name
password : password
database : A

dummy:
adapter  : mysql
username : user-name
password : password
database : B

Consider two models:
a) Animal – table = animals in Database A.
b) Person – table – persons in Database B.

Usually the classes will be like follows:

class Person << ActiveRecord::Base
………
end

class Animal << ActiveRecord::Base
………
end

Now for Rails to know that it has to connect to DATABASE B when it needs to fetch data from Table “persons”, you hust have to add a line in the Person model. i.e:
establish_connection :dummy
where,
establish_connection  Рmethod which connects to the Database specified in the environment
dummy – environment name

Thats it.
Try the following in console:

>> Animal.connected? #=> false
>> Animal
>> Animal.connected? #=> true

>> Person.connected? #=> false
>> Person
>> Person.connected? #=> true

connected? is the method which returns whether or not RAILS has connected with the DATABASE in which the table related to this MODEL exists.
Initailly when the RAILS application loads (CONSOLE starts), it has no DB connections.
When u say Animal, it fetches the schema of animals table by connecting to Database A.
So, after that , if u say Animal.connect? , it checks whether or not RAILS has connected to the DB in which the table related to Aniaml(animals) exists.
As it has already connected in the above step, the answer is TRUE.

Same is the case with Person.

The point to observe is that, Before working on the Model, Rails executes “establish_connection” method.
If it is not specified in the model, it connects to d DB as specified in the current ENVIRONMENT, if its not already connected.

But if we want o connect to a different DB as in the case with Person model, we override this, by specifying the DATABASE NAME to be connected by specifying the environment name in the “establish_connection” as the 1’st line of the Class.

Hope this is useful.

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rake migrate in production environment

May 27, 2009 at 9:57 am (tasks) (, , , )

This is the code in the command line to run the database migrations in Production environment, infact any defined environment in database.yml

rake db:migrate RAILS_ENV=”name_of_the_environment”
Keep in mind that, the file called name_of_the_environment.rb should already be available in config/environments.rb

This can be just a copy of the existing files like config/development.rb

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twitter user tweets

May 13, 2009 at 6:02 pm (Twitter) (, , )

The following code describes the way we can fetch the tweets of the user.

I just came across this wonderful GEM to integrate TWITTER in rails.
This is called Twitter4r

Its API is available at API

Its such an easy gem to use. I thought i shall throw the steps needed for the most regular task needed….Getting the tweets of a particular user.

Install twitter4r.

require “twitter”

twitter = Twitter::Client.new(:login=>”login_name”,:password=>”password”)tweets = twitter.timeline_for(:user,:id=>”user_name_of_user_whose_tweets_you_need”)
The result is an array
The important attributes of this array-elements are:
@text – The text of this tweet
@user – The user who posted this tweet
@user.name – name of the user who posted this tweet.
@user.screen_name – screen name of the user who posted this tweet

Hope this is useful.

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passing optional parameters to partials

May 13, 2009 at 4:38 pm (Partials) (, , , )

Today i got an instance where i want to use a parameter in the partial, which may or may not be passed.This partial is called from two places, one which passes while other does not pass a specific parameter. The partial just displays the parameter value.

<%= passed_parameter %>

If an instance variable is used, the <%= … %> will result in an empty string if the instance variable (@abcdef) id not defined. But if its a local variable like the one passed to the partials using LOCALS, it will throw an error.

In these cases, the best thing to do is, add this at the first line of the PARTIAL.

<% passed_parameter ||= DEFAULT_VALUE %>

This has to be done for whatever optional parameters might be used.

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