Nested strong parameters in rails - AssociationTypeMismatch MYMODEL expected, got ActionController::Parameters()

Are you using accepts_nested_attributes_for :books on your project model? If so, instead of "books", the key should be "books_attributes".

def project_params
  params.permit(:id, :complete, :false, :private, books_attributes: [:id, :name])
end

I'm using Angular.js & Rails & Rails serializer, and this worked for me:

Model:

• has_many :features
• accepts_nested_attributes_for :features

ModelSerializer:

• has_many :features, root: :features_attributes

Controller:

• params.permit features_attributes: [:id, :enabled]

AngularJS:

• ng-repeat="feature in model.features_attributes track by feature.id

My solution to this using ember.js was setting the books_attributes mannualy.

In controller:

def project_params      
  params[:project][:books_attributes] = params[:project][:books_or_whatever_name_relationships_have] if params[:project][:books_or_whatever_name_relationships_have]
  params.require(:project).permit(:attr1, :attr2,...., books_attributes: [:book_attr1, :book_attr2, ....])

end

So rails checks and filters the nested attributes as it expected them to come