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Ruby ActiveRecord Basics Migrations and Relationships Relationships

Antoine Boillot
Antoine Boillot
10,466 Points
Posted by Antoine Boillot
Antoine Boillot
10,466 Points

ActiveRecord : database relationship setup vs foreign keys setup ?

Hi All,

In the ActiveRecord courses, Migration section, Hampton is setting up the following table as below :

class CreateTimeEntries < ActiveRecord::Migration
  def change
    create_table :time_entries do |t|
        t.float :time
        t.belongs_to :customer
        t.belongs_to :employee
        t.timestamps
    end
  end
end

He then defines in the relevant model their relationship with already existing tables, see below :

class TimeEntry < ActiveRecord::Base
    belongs_to :customer
    belongs_to :employee
end

Aren't those lines redondant ? Could we avoid typing one or the other ?

#in table setup
t.belongs_to :customer
t.belongs_to :employee

#in the relevant model
belongs_to :customer
belongs_to :employee

Complementary question : If the lines in the table setup are here to defines foreign keys, how come then that we need to define the relationship in the model as well? I thought foreign keys were by themselves defining such relationship.

Hope I'm clear.

Thanks a lot for your help !

1 Answer

Maciej Czuchnowski
Maciej Czuchnowski
36,441 Points
Maciej Czuchnowski
Maciej Czuchnowski
36,441 Points

The belongs_to in the migration is not the same thing as the belongs_to in the model. In the migration it adds the _id column of the associated model (it's the same as writing add_reference :time_entries, :customer). When used in the model, the belongs_to association lets you streamline some code. This - 1 Why Associations? - explains it nicely:

http://guides.rubyonrails.org/association_basics.html