custom_predicate_with_dependency.rb

require 'dry-validation'

User = Struct.new(:name, :email)

class UserRepository
  USERS = []

  def create(attributes)
    USERS << User.new(*attributes.values_at(:name, :email))
  end

  def find_by_email(email)
    USERS.find { |user| user.email == email }
  end
end

UserSchema = Dry::Validation.JSON do
  configure do
    option :uniqueness_finder

    def self.messages
      super.merge(en: { errors: { unique?: 'must be unique' } })
    end

    def unique?(value)
      uniqueness_finder.call(value).nil?
    end
  end

  required(:name).required
  required(:email).filled(:unique?)
end

user_repo = UserRepository.new
user_validator = UserSchema.with(uniqueness_finder: user_repo.method(:find_by_email))
user_repo.create(name: 'John', email: '[email protected]')

user_validator.call(
  name: 'John',
  email: '[email protected]'
)
# Failure
user_validator.call(
  name: 'Jill',
  email: '[email protected]'
)