In Ruby on Rails, Active Record is the M in MVC - the model. Model is the layer of the system which represents business data and logic. Active Record facilitates the creation and the use of business objects whose data requires persistent storage to a database. In some cases, however, you may want to use databases in Rails applications without the Model. What you can do is using the pg gem to directly manipulate the data you need.

Create a database connection

If the database is not declared in Rails app’s config/database.yml file, you won’t be able to use the establish_connection method in ActiveRecord. But you can still use the pg gem to directly connect to the database.

1
2
3
4

# page_controller.rb

conn = PGconn.connect('<server>', <port>, "", "", '<database>', "<user>", "<password>")
render :plain => "Oops, database currently not accessible" unless conn

Query data with RAW SQL

1
2
3
4
5
6

# page_controller.rb

@page_records = []
result = conn.exec('SELECT * FROM <table> ORDER BY id DESC')

@page_records = result

The result is an array of Hash. For example:

1
2
3
4

[
  { "title" => "my page 1", "author" => "Tom", "created_at" => "2016-06-08" }
  { "title" => "my page 2", "author" => "Nancy", "created_at" => "2016-06-09" }
]

Use the data in View

When using the queried data in Rails View, need to note you need use record['title'] instead of record.title, which is different from using the data generated from a Rails Model.

1
2
3
4
5
6
7
8

# page.erb

<div>
    <% @page_records.each do |record| %>
        <p> <%= record['title'] %> </p>
        <p> <%= record['author'] %> </p>
    <% end %>
</div>

As a note in the end

Although the above approach works in Rails, I’d say a more decent way to handle the need of getting data from multiple databases, is creating a crafted Model. The process will need:

• Add the configurations of all your external databases in Rails app
• Create a Model file

1
2
3

class Pages < ActiveRecord::Base

end

• Use establish_connection in the Model
• When your table name doesn’t match Rails convention, set the table name with self.table_name = '<your table name>'
• Create methods in the Model to fetch the data you need