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

I'm rendering a model and it's children Books in JSON like so: ``` {"id":2,"complete":false,"private":false, "books" [{ "id":2,"name":"Some Book"},..... ``` I then come to update this model by passing the same JSON back to my controller and I get the following error: ActiveRecord::AssociationTypeMismatch (Book (#2245089560) expected, got ActionController::Parameters(#2153445460)) In my controller I'm using the following to update: ``` @project.update_attributes!(project_params) private def project_params params.permit(:id, { books: [:id] } ) end ``` No matter which attributes I whitelist in `permit` I can't seem to save the child model. Am I missing something obvious? Update - another example: Controller: ``` def create @model = Model.new(model_params) end def model_params params.fetch(:model, {}).permit(:child_model => [:name, :other]) end ``` Request: ``` post 'api.address/model', :model => { :child_model => { :name => "some name" } } ``` Model: ``` accepts_nested_attributes_for :child_model ``` Error: expected ChildModel, got ActionController::Parameters Tried this method to no avail: http://www.rubyexperiments.com/using-strong-parameters-with-nested-forms/