Polymorphism

Polymorphism

Polymorphism allows Ohm models to be derived from, subclassed and specialized. Subclasses of an Ohm model may add attributes, collections and indices to the root model. When the root is queried using find, a polymorphic collection of subclasses of the root class is returned.

Example

class User < Ohm::Model
  polymorphic(true)
  attribute :name
  index :name
end

class SuperUser < User
  attribute :kernel
  index :kernel
end

Here SuperUser derives from User and adds the indexed attribute kernel. The line polymorphic(true) flags the model User as the root of a polymorphic tree of model classes.

Model Root

All models that have subclasses are deemed polymorphic. The model root is the superclass of all its polymorphs. The roots are your application classes that derive from Ohm::Model:

>> User.root
=> User
>> SuperUser.root
=> User

>> User.create( name:'jojo' )
=> #<User:1 name="jojo">
>> SuperUser.create( name:'lenny', kernel:'debian' )
=> #<SuperUser:2 name="lenny" kernel="debian">

The all collection for the model root contains all subclasses of the root:

>> User.all.map(&:name)
=> ["jojo", "lenny"]
>> User.all.map(&:class)
=> [User, SuperUser]

Whereas the subclass all collections contain only objects of that subclass and its descendants:

>> SuperUser.all.map(&:name)
=> ["lenny"]

In effect, we persist a hidden _type attribute with all subclass instances, and Subclass.all is the name of the index on _type.

Finding Subclasses

When you perform a find, you specify the model root or the first descendant that has all of the indices that you are searching on:

>> User.find( name:'lenny' ).first
=> #<SuperUser:2 name="lenny" kernel="debian">
>> SuperUser.find( kernel:'debian' ).first
=> #<SuperUser:2 name="lenny" kernel="debian">

>> User.find( kernel:'centos' )
=> Ohm::Model::IndexNotFound  # error    

Setting the Base Class

What if your model roots derive from a common application base class, ModelBase, rather than Ohm::Model? You set the base class like this:

class ModelBase < Ohm::Model
  self.base = self
end

class User < ModelBase; ... end
class SuperUser < User; ... end