Change schema definition DSL

[vladfaust]

class User
  include Core::Schema
  include Core::Query
  include Core::Validation

  schema :users, :id do
    type posts : Array(Post), foreign_key: :author_id
    type username : String, validate: {size: (1..32)}}
    type email : String, validate: {regex: /@/}
    type created_at : Time = db_default
  end
end

* db_default marks property value set on DB side