can't delete object due to foreign key constraint

You would need to remove the Identity that references the user first. Then you can delete the user.. By default the foreign key is doing a restrict so you cannot delete the user if anything references to it.

if you would like use Rails to handle destroying the identity you can do

class User < ActiveRecord::Base
  has_many :identities,  dependent: :destroy 

  ......

 end 

Which would cause Rails to destroy all the dependent records.

But as you are using Foreign keys, you can adjust your migration to set cascade deletes

 add_foreign_key :identities, :users, on_delete: :cascade

Assuming rails 4.2 which has native support

An easy solution is to simply cascade-delete the records in the associated table, which can be done through active record, like so:

user.rb

class User < ActiveRecord::Base
  has_many :identities, dependent: :destroy

  # rest of user class
end

Check out the documentation pertaining to has_many for more info.