UPD. Try to use following approach (tested in separate file):
@params = ActionController::Parameters.new(
applicant: {answers_attributes: {
"0" => {question_id: 10, id: 110, properties: {a: "b", c: "d"}},
"1" => {question_id: 20, id: 120, properties: {m: "n", o: "p"}}
}})
def applicant_params
#properties should be [:a, :c, :m, :o]
properties = []
@params[:applicant][:answers_attributes].values.each do |answer|
properties |= answer[:properties].keys
end
@params.require(:applicant).permit(answers_attributes:
[:question_id, :id, properties: properties])
end
BTL. There is pretty good article on working with hstores. And some general things on using hstore in Rails 4.