One can use :conditions option to validates_uniqueness_of to limit the scope of validation:

            Dictionary.validates_uniqueness_of(:name,
              conditions: -> { Dictionary.where(:year.gte => 1900, :year.lt => 2000) })

This allows dictionary to be created with years 950 and 1950:

              Dictionary.create!(name: "French-English", year: 950)
              Dictionary.create!(name: "French-English", year: 1950)

... but not the other way around:

              Dictionary.create!(name: "French-English", year: 1950)
              Dictionary.create!(name: "French-English", year: 950)

One might expect the specified conditions to be applied to the model instance being validated, but this is not the case: conditions only apply to the database query. For avoidance of confusion, this behavior should be documented.

AR works the same way as Mongoid.