Connecting and Associating multiple databases and their tables in rails(Part 1)

There are several steps that needs to follow to connect with multiple database and create association between two different tables of two different databases

First you need to modify your database.yml

1.)database.yml:

main_development:

  # development configuration goes here

secondary_development:

  # development configuration to external database goes here

# same for test and production...

2.)Models:

If you have two different databases the you would have multiple tables in each database and off-Course multiple models in each db now the question is how to establish the connection between two models of two different database.

But Rails provide it in easy way

#Model in main_database

To manually connect each model of main_database with secondary database you need to establish a connection to that database like:

class Astronaut < ActiveRecord::Base

  establish_connection "nasa_#{Rails.env}"

end

or

create a model which establishes the connection to secondary database and now each model can inherit or not to that model

class NasaTable < ActiveRecord::Base

  self.abstract_class = true  

  establish_connection "nasa_#{Rails.env}"

end

Question: Why the hell are you creating a abstract class and what is the problem if I would write establish connection to each model in which I needs to connect to the secondary database or main_database.

Answer: if you do this you will end up with multiple connection pools within two databases so create only one connection.

 

The self.abstract_class = true tells Active Record to not look up for a table, since this class is only used to add custom settings we don’t need any database table for it.

class Astronaut < NasaTable

self.table_name='astronauts'

  has_many :missions

  has_many :shuttles, through: :missions

end

class Shuttle < NasaTable

self.table_name='shuttles'

  has_many :missions

  has_many :astronauts, through: :missions

end

class Mission < NasaTable

self.table_name='missions'

  belongs_to :astronaut

  belongs_to :shuttle

end

# test

shuttle = Shuttle.create name: "Endeavor"

shuttle.astronauts << Astronaut.create(name: "Mark Kelly")

assert_equal 1, shuttle.reload.astronauts.count

self.table_name tells the name of the database for that particular model it is not really not necessary.