Rails: Validate unique combination of 3 columns

Question

Hi I wan't to validate the unique combination of 3 columns in my table.

Let's say I have a table called cars with the values :brand, :model_name and :fuel_type.

What I then want is to validate if a record is unique based on the combination of those 3. An example:

    brand    model_name    fuel_type
    Audi     A4            Gas
    Audi     A4            Diesel
    Audi     A6            Gas

Should all be valid. But another record with 'Audi, A6, Gas' should NOT be valid.

I know of this validation, but I doubt that it actually does what I want.

    validates_uniqueness_of :brand, :scope => {:model_name, :fuel_type}

Answer 1

There is a syntax error in your code snippet. The correct validation is :

validates_uniqueness_of :car_model_name, :scope => [:brand_id, :fuel_type_id]

or even shorter in ruby 1.9.x:

validates_uniqueness_of :car_model_name, scope: [:brand_id, :fuel_type_id]

with rails 4 you can use:

validates :car_model_name, uniqueness: { scope: [:brand_id, :fuel_type_id] }

with rails 5 you can use

validates_uniqueness_of :car_model_name, scope: %i[brand_id fuel_type_id]

Answer 2

Depends on your needs you could also to add a constraint (as a part of table creation migration or as a separate one) instead of model validation:

add_index :the_table_name, [:brand, :model_name, :fuel_type], :unique => true

Adding the unique constraint on the database level makes sense, in case multiple database connections are performing write operations at the same time.

Answer 3

To Rails 4 the correct code with new hash pattern

validates :column_name, uniqueness: {scope: [:brand_id, :fuel_type_id]}