rails creating relationships between 2 existing models

Question

I have 2 models:

Account
Profile

creating these models created 2 tables in the database:

accounts
profiles

now I want to add a relationship:

each account can have many profiles
each profile belongs to one account

I ran the following command:

rails g migration AddAccountToProfiles account:references

which created the following migration:

class AddAccountToProfiles < ActiveRecord::Migration
  def change
    add_reference :profiles, :account, index: true, foreign_key: true
  end
end

now I'm a little confused:

why does the migration say :profiles and :account? Shouldn't it be :accounts (plural)?

also, after (or before) creating this migration, I have to add belongs_to and has_many in the appropriate model class right?

as a side question, is there a way to add belongs_to and has_many in the models, and from that information have rails generate the appropriate migration without me manually creating a migration with the 'rails g migration ...' command?

@lusketeer I actually read that answer and that's where I learned to use the migration command, but I want to understand the details of why singular/plural names are generated — Abdul Ahmad, Jun 03 '16 at 14:16
as per [rails documentation](http://edgeguides.rubyonrails.org/active_record_migrations.html#highlighter_889766), the command will generate the migration file above, since you specified user:references, then it assumes to create `user_id` on `profiles` table, but you still need to add the relationships in the corresponding model files. when we use `:accounts` in migration file, it's referring to the table, `:account` is used as the name of the foreign key to be added onto the table along with suffix `_id` — lusketeer, Jun 03 '16 at 14:22

score 4 · Accepted Answer · edited May 23 '17 at 11:51

As per rails documentation, the command

rails g migration AddAccountToProfiles account:references

will generate the migration file below

class AddAccountToProfiles < ActiveRecord::Migration
  def change
    add_reference :profiles, :account, index: true, foreign_key: true
  end
end

Since you specified account:references, then it assumes to create account_id on profiles table, but you still need to add the relationships in the corresponding model files.

When we use :accounts in migration file, it's referring to the table in the database, :account is used as the name of the foreign key to be added onto the table along with suffix _id

Also relevant information here

score 1 · Answer 2 · answered Jun 03 '16 at 14:30

The migration is correct, because a profile only belongs to one account. It should not be 'accounts'. The migration will place a account_id column onto the profiles table in order to make that connection.

After the migration, you still need to add has_many and belongs_to. In Rails, when defining a relationship, there are generally two steps 1) create the database migration 2) define the relation on the model class itself. You need to have both. In this case, Rails is looking for the account_id column on a profile (the default foreign key) to make the relationship between the two models.

And as for your last question, no, there is not a way to generate migrations after defining a has_many. You can use Rails generators to create the model itself rails generate model ModelName and define the relationship in that model; that will add the correct belongs_to and has_many into the generated model along with the migration. But in practice, it's generally better to create the migration and manually add belongs_to and has_many as needed so there's less of a chance of missing something.

rails creating relationships between 2 existing models

2 Answers2