Example of dry-validation predicate for "unique within some scope"

example.rb

UserSchema = Dry::Validation.Schema do
  configure do
    # Hard-coding the custom error messages
    def self.messages
      Dry::Validation::Messages.default.merge(
        en: {errors: {unique_within_account?: 'is not unique within this account'}}
      )
    end

    # Here's the custom predicate
    def unique_within_account?(account_id, user_name)
      # Faking this out here. IRL you'd use an injected option or a hard-coded
      # lookup to some account/user data.
      !user_name.include?(account_id.to_s)
    end
  end

  required(:account_id).filled(:int?)
  required(:name).filled

  rule(name: [:account_id, :name]) do |account_id, name|
    name.unique_within_account?(account_id)
  end
end

valid_input = {account_id: 1, name: "my_name"}
# UserSchema.(valid_input)
# => #<Dry::Validation::Result output={:account_id=>1, :name=>"my_name"} errors={}>

invalid_input = {account_id: 99, name: "my_name_99"}
UserSchema.(invalid_input)
# => #<Dry::Validation::Result output={:account_id=>99, :name=>"my_name_99"} errors={:name=>["is not unique within this account"]}>