Validate uniqueness of multiple columns

You can use validates to validate uniqueness by one column:

validates :user_id, uniqueness: {scope: :friend_id}

The syntax for the validation by multiple columns is similar, but you should provide an array of fields instead:

validates :attr, uniqueness: {scope: [:attr1, ... , :attrn]}

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However, validation approaches shown above have a race condition and can’t ensure consistency. Consider the following example:

1. database table records are supposed to be unique by n fields;

2. multiple (two or more) concurrent requests, handled by separate processes each (application servers, background worker servers or whatever one is using), access database to insert the same record in table;

3. each process in parallel validates if there is a record with the same n fields;

4. validation for each request is passed successfully, and each process creates a record in the table with the same data.

To avoid this kind of behaviour, one should add a unique constraint to db table. You can set it with add_index helper for one (or multiple) field(s) by running the following migration:

class AddUniqueConstraints < ActiveRecord::Migration
  def change
   add_index :table_name, [:field1, ... , :fieldn], unique: true
  end
end

Caveat : even after you've set a unique constraint, two or more concurrent requests will try to write the same data to db, but instead of creating duplicate records, this will raise an ActiveRecord::RecordNotUnique exception, which you should handle separately:

begin
# writing to database
rescue ActiveRecord::RecordNotUnique => e
# handling the case when record already exists
end 

You can scope a validates_uniqueness_of call as follows.

validates_uniqueness_of :user_id, :scope => :friend_id