Constructor Types (deprecated)

Constructor types were removed in v0.5.0. Use transform_types and transform_keys as a replacement.

Your struct class can specify a constructor type, which uses hash schemas to handle attributes in .new method. By default :permissive constructor is used.

To set a different constructor type simply use constructor_type setting:

class User < Dry::Struct
  constructor_type :strict

  attribute :name, Types::Strict::String
  attribute :age, Types::Strict::Integer
end

User.new(name: "Jane", age: 31)
# => #<User name="Jane" age=31>

User.new(name: "Jane", age: 31, unexpected: "attribute")
# Dry::Struct::Error: [User.new] unexpected keys [:unexpected] in Hash input

class Admin < Dry::Struct
  constructor_type :schema

  attribute :name, Types::Strict::String.default('John Doe')
  attribute :age, Types::Strict::Integer
end

Admin.new(name: "Jane")        #=> #<User name="Jane" age=nil>
Admin.new(age: 31)             #=> #<User name="John Doe" age=31>
Admin.new(name: nil, age: 31)  #=> #<User name="John Doe" age=31>
Admin.new(name: "Jane", age: 31, unexpected: "attribute")
  #=> #<User name="Jane" age=31>

Common constructor types include:

  • :permissive - the default constructor type, useful for defining structs that are instantiated using data from the database (ie results of a database query), where you expect all defined attributes to be present and it’s OK to ignore other keys (ie keys used for joining, that are not relevant from your domain structs point of view). Default values are not used otherwise you wouldn’t notice missing data.
  • :schema - missing keys will result in setting them using default values, unexpected keys will be ignored.
  • :strict - useful when you do not expect keys other than the ones you specified as attributes in the input hash
  • :strict_with_defaults - same as :strict but you are OK that some values may be nil and you want defaults to be set
  • :weak and :symbolized - don’t use those with dry-struct, and instead use dry-validation to process and validate attributes, otherwise your struct will behave as a data validator which raises exceptions on invalid input (assuming your attributes types are strict)