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There are distinctive reasons why you should think about having various databases in your Ruby on Rails application. In my situation, I expected to store large quantities of data.

Consider default database is MySQL. In our application database.yml file write connections for MySQL in normal way. After that, for connecting postgresql in the same application we need to create custom files.

Create the custom database files to connect postgresql

We’re going to set up a second database called “Stats”

First of all, create the file config/database_stats.yml and populate it as you do with the primary database’s config file.

Your file will look something like this:

development:
  adapter: postgresql
  encoding: utf8
  reconnect: false
  database: db_info_development
  pool: 5
  host: localhost
  username: postgres
  password:

We’re now going to create a directory that will hold the schema and all the migrations of the Stats database.

Create directory with name db_stats in the rails root and copy the structure as mentioned below

–db
  –migrate
  schema.rb
  seeds.rb
–db_stats
  –migrate
  schema.rb
  seeds.rb

The created files should be empty.

Add Rake Tasks

For handling stats database, we need to write custom tasks for creation, migrations and other functionalities.

Create a file lib/tasks/db_stats.rake with the below content

namespace :stats do

  namespace :db do |ns|

    task :drop do
      Rake::Task[“db:drop”].invoke
    end

    task :create do
      Rake::Task[“db:create”].invoke
    end

    task :setup do
      Rake::Task[“db:setup”].invoke
    end

    task :migrate do
      Rake::Task[“db:migrate”].invoke
    end

    task :rollback do
      Rake::Task[“db:rollback”].invoke
    end

    task :seed do
      Rake::Task[“db:seed”].invoke
    end

    task :version do
      Rake::Task[“db:version”].invoke
    end

    namespace :schema do
      task :load do
        Rake::Task[“db:schema:load”].invoke
      end

      task :dump do
        Rake::Task[“db:schema:dump”].invoke
      end
    end

    namespace :test do
      task :prepare do
        Rake::Task[“db:test:prepare”].invoke
      end
    end

    # append and prepend proper tasks to all the tasks defined here above
    ns.tasks.each do |task|
      task.enhance [“stats:set_custom_config”] do
        Rake::Task[“stats:revert_to_original_config”].invoke
      end
    end
  end

  task :set_custom_config do
    # save current vars
    @original_config = {
      env_schema: ENV[‘SCHEMA’],
      config: Rails.application.config.dup
    }

    # set config variables for custom database
    ENV[‘SCHEMA’] = “db_stats/schema.rb”
    Rails.application.config.paths[‘db’] = [“db_stats”]
    Rails.application.config.paths[‘db/migrate’] = [“db_stats/migrate”]
    Rails.application.config.paths[‘db/seeds’] = [“db_stats/seeds.rb”]
    Rails.application.config.paths[‘config/database’] = [“config/database_stats.yml”]
  end

  task :revert_to_original_config do
    # reset config variables to original values
    ENV[‘SCHEMA’] = @original_config[:env_schema]
    Rails.application.config = @original_config[:config]
  end
end

 

Once all of this setup is done, we can create the stats database and run its first migration:

$ rake stats:db:create
$ rake stats:db:migrate

This will generate the Stats database schema file in db_stats/schema.rb.

Add a custom migration generator
We cannot use rails generator because the path hardcodes the db/migrate. Therefore, we need to have custom generator to create all migrations for the stats database.

Create the generator in lib/generators/stats_migration_generator.rb and paste the following code:

require ‘rails/generators/active_record/migration/migration_generator’

class StatsMigrationGenerator < ActiveRecord::Generators::MigrationGenerator
source_root File.join(File.dirname(ActiveRecord::Generators::MigrationGenerator. instance_method(:create_migration_file) .source_location.first), “templates”)

  def create_migration_file
    set_local_assigns!
    validate_file_name!
    migration_template @migration_template, “db_stats/migrate/#{file_name}.rb”
  end
end

With all of this in place, we can now generate migrations for the Stats database:

$ rails g stats_migration migration_name

This migration file gets created in the stats database migrate directory db_stats/migrate.

After that, run the following command to migrate.

$ rake stats:db:migrate

Finalize connection and models for PostgreSQL
We’re almost done. Add a new initializer file config/initializers/db_stats.rb and paste the following:

# save stats database settings in global var

DB_STATS = YAML::load(ERB.new(File.read(Rails.root.join(“config”,”database_stats.yml”))).result)[Rails.env]

Configuring mongodb:

Mongoid gem is an ODM (Object-Document-Mapper) framework for MongoDB in Ruby.

gem ‘mongoid’, ‘~> 5.1.0’

You can generate a config file by executing the generator. Mongoid will then handle everything else from there.

$ rails g mongoid:config

Then it creates mongoid.yml as below

development:
    clients:
        default:
            database: mongoid
            hosts:
                -localhost:27017

Now in our application we have successfully integrated the MySQL, postgresql and mongodb

Consider three models – User, Product, Image. Using these three models will help us connect different databases in our application.

User model with MySQL connection

class User < ActiveRecord::Base
end

$ rails g migration create_users field_names

rake db:migrate

This migration file gets created under db/migrate and it will store under MySQL database.

class Product < ActiveRecord::Base
    establish_connection DB_STATS
end

$ rails g stats_migration create_products
      create  db_stats/migrate/20151201191642_create_products.rb

This migration file gets created in the stats database migrate directory db_stats/migrate.

class Image
    include Mongoid::Document

    field :name, type: String
    field :description, type: String
end

No need for running any migrations for mongodb since it is ODM. This data is created under MongoDB.

Feel free to contact us if you have any query.

Katneni Naga Sai Tejaswi
Sr. Software Developer
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