Diving into ActiveRecord sources i found that
ActiveRecord::Relation covers 'create' with 'scoping' method.
ActiveRecord::Persistance 'create' calls 'initialize' from ActiveRecord::Core.
ActiveRecord::Core 'initialize' calls 'populate_with_current_scope_attributes'
This method declared in ActiveRecord::Scoping uses 'scope_attributes' declared in ActiveRecord::Scoping::Named.
scope_attributes creating relation 'all' and calls 'scope_for_create' on it.
'ActiveRecord::Relation's 'scope_for_create' uses only 'where_values_hash' from current_scope that does not contain rules like 'related_models.subject_type' (this values are contained in where_clauses). So we need to have simple key-value wheres to be used with 'create' on ActiveRecord::Relation. But ActiveRecord not clever enough to know that 'some_field' in where clause should be used with join table.
I found it can be implemented only by accessing where options with self.class.current_scope.where_clauses in 'before_create' on MyModel, parsing them and setting up attributes.
class MyModel < AR::Base
before_create :create_default_node
def create_default_node
clause = self.class.current_scope.where_clauses.detect{|clause| clause =~ /\`related_models\`.\`some_field\`/}
value = clause.scan(/\=.+\`([[:word:]]+)\`/).flatten.first
self.create_node(:some_field => value)
end
end
But it is so dirty, then i decided to find simpler solution and inverted dependency as described in Railscast Pro #394, moved RelatedModel functionality to MyModel with STI. Actually i needed such complicated relation creation because RelatedModel had some functionality common for all models (acts as tree). I decided to delegate 'ancestors' and 'children' to RelatedModel. Inverting dependency solved this problem.
class MyModel < AR::Base
acts_as_tree
belongs_to :subject, :polymorphic => true
end
class MyModel2 < MyModel
end
class RelatedModel < AR::Base
# has :some_field
has_one :my_model, :as => :subject, :dependent => :destroy
end
MyModel.create{|m| m.subject = RelatedModel.create(:some_field => "chachacha")}
MyModel.ancestors # no need to proxy relations