A migration to add unique constraint to a combination of columns

Question

What I need is a migration to apply unique constraint to a combination of columns. i.e. for a people table, a combination of first_name, last

Answer 1

According to howmanyofme.com, "There are 46,427 people named John Smith" in the United States alone. That's about 127 years of days. As this is well over the average lifespan of a human being, this means that a DOB clash is mathematically certain.

All I'm saying is that that particular combination of unique fields could lead to extreme user/customer frustration in future.

Consider something that's actually unique, like a national identification number, if appropriate.

(I realise I'm very late to the party with this one, but it could help future readers.)

Answer 2

In the typical example of a join table between users and posts:

create_table :users
create_table :posts

create_table :ownerships do |t|
  t.belongs_to :user, foreign_key: true, null: false
  t.belongs_to :post, foreign_key: true, null: false
end

add_index :ownerships, [:user_id, :post_id], unique: true

Trying to create two similar records will throw a database error (Postgres in my case):

ActiveRecord::RecordNotUnique: PG::UniqueViolation: ERROR:  duplicate key value violates unique constraint "index_ownerships_on_user_id_and_post_id"
DETAIL:  Key (user_id, post_id)=(1, 1) already exists.
: INSERT INTO "ownerships" ("user_id", "post_id") VALUES ($1, $2) RETURNING "id"

e.g. doing that:

Ownership.create!(user_id: user_id, post_id: post_id)
Ownership.create!(user_id: user_id, post_id: post_id)

Fully runnable example: https://gist.github.com/Dorian/9d641ca78dad8eb64736173614d97ced

db/schema.rb generated: https://gist.github.com/Dorian/a8449287fa62b88463f48da986c1744a

Answer 3

add_index :people, [:firstname, :lastname, :dob], :unique => true

Answer 4

Hi You may add unique index in your migration to the columns for example

add_index(:accounts, [:branch_id, :party_id], :unique => true)

or separate unique indexes for each column

Answer 5

You may want to add a constraint without an index. This will depend on what database you're using. Below is sample migration code for Postgres. (tracking_number, carrier) is a list of the columns you want to use for the constraint.

class AddUniqeConstraintToShipments < ActiveRecord::Migration
  def up
    execute <<-SQL
      alter table shipments
        add constraint shipment_tracking_number unique (tracking_number, carrier);
    SQL
  end

  def down
    execute <<-SQL
      alter table shipments
        drop constraint if exists shipment_tracking_number;
    SQL
  end
end

There are different constraints you can add. Read the docs

Answer 6

For completeness sake, and to avoid confusion here are 3 ways of doing the same thing:
Adding a named unique constraint to a combination of columns in Rails 5.2+

Let's say we have Locations table that belongs to an advertiser and has column reference_code and you only want 1 reference code per advertiser. so you want to add a unique constraint to a combination of columns and name it.

Do:

rails g migration AddUniquenessConstraintToLocations

And make your migration look either something like this one liner:

class AddUniquenessConstraintToLocations < ActiveRecord::Migration[5.2]
  def change
    add_index :locations, [:reference_code, :advertiser_id], unique: true, name: 'uniq_reference_code_per_advertiser'
  end
end

OR this block version.

class AddUniquenessConstraintToLocations < ActiveRecord::Migration[5.2]
  def change
    change_table :locations do |t|
     t.index ['reference_code', 'advertiser_id'], name: 'uniq_reference_code_per_advertiser', unique: true
    end
  end
end

OR this raw SQL version

class AddUniquenessConstraintToLocations < ActiveRecord::Migration[5.2]
  def change
      execute <<-SQL
          ALTER TABLE locations
            ADD CONSTRAINT uniq_reference_code_per_advertiser UNIQUE (reference_code, advertiser_id);
        SQL
  end
end

Any of these will have the same result, check your schema.rb