How to add foreign key in rails migration with different table name

Question

How can I assign different table name with adding foreign key. for e.g

I have a model like

class MyPost < ActiveRecord::Base
  has_many :comments, class_name: PostComment
end

class PostComment < ActiveRecord::Base
  belongs_to :post, class_name: MyPost
end

Now i want to change my migration file like this:

class CreatePostComments < ActiveRecord::Migration
  def change
    create_table :post_comments do |t|
     t.belongs_to :post, index: true
     t.timestamps null: false
    end
    add_foreign_key :post, :class_name => MyPost
  end
end

But it is not working. Migration is getting cancelled. How do I change my migration file to work with my model structure.

`t.belongs_to :post, index: true` created the foreign key for you. Why again you are trying ? I am not getting what you are trying to do.. — Arup Rakshit, Aug 11 '15 at 08:15

score 48 · Answer 1 · edited Jan 22 '19 at 09:08

48

You can pass in options for the foreign key as following:

class CreatePostComments < ActiveRecord::Migration
  def change
    create_table :post_comments do |t|
      t.references :post, foreign_key: { to_table: :my_posts }, index: true
      t.timestamps null: false
    end
  end
end

This is also true for the index option if you like to add a unique constraint:

t.references :post, foreign_key: { to_table: :my_posts }, index: { unique: true}

By the way, references is an alias for belongs_to, or to be more exact, belongs_to is an alias for references.

See the details in the implementation rails 5.0.rc2 & rails 4.2

edited Jan 22 '19 at 09:08

ray

5,454
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answered Jun 23 '16 at 06:43

Stefan Staub

689
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`index` is `true` by default. `index: true` is unnecessary. – developer033 Feb 07 '17 at 01:55
8

[`index` is `true` by default from Rails 5](http://api.rubyonrails.org/classes/ActiveRecord/ConnectionAdapters/SchemaStatements.html#method-i-add_reference). [`index` is `false`by default on previous Rails versions](http://api.rubyonrails.org/v4.2/classes/ActiveRecord/ConnectionAdapters/SchemaStatements.html#method-i-add_reference). – cbliard Jul 20 '17 at 08:25
1

I'd modify it to `t.references :post, foreign_key: { to_table: :my_posts }, null:false`, cause it won't add `null: false` by default – Alexander Gorg May 12 '20 at 14:34

score 18 · Answer 2 · answered Nov 12 '15 at 11:29

It should look like this:

class CreatePostComments < ActiveRecord::Migration
  def change
    create_table :post_comments do |t|
     t.belongs_to :post, index: true
     t.timestamps null: false
    end
    add_foreign_key :post_comments, :my_posts, column: :post_id
  end
end

Take a look at the documentation: http://apidock.com/rails/ActiveRecord/ConnectionAdapters/SchemaStatements/add_foreign_key

You use the column option when the column is named differently.

How to add foreign key in rails migration with different table name

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