error_type_one.cr

abstract class Form(T)
  def go(object : T)
  end
end

class Thing
end

class ThingForm < Form(Thing)
end

ThingForm.new.go("foo")
# no overload matches 'ThingForm#go' with type String

migration.cr

class GenerateUsernamesForUsers::V20190120141952 < LuckyRecord::Migrator::Migration::V1
  class User < BaseModel
    table :users do
      column username : String?
    end
  end

  def migrate
    UserQuery.all.each do |user|
      new_username = user.email.split("@").first
      User::BaseForm.update!(user, username: new_username)
      puts "Generated username #{new_username} for #{user.email}"
    end
  end

  def rollback
  end
end

# no overload matches 'GenerateUsernamesForUsers::V20190120141952::User::BaseForm.update!', record: User, username: String