I have a model Sensor with has_many and accepts_nested_attributes_for relationships to another model Watch. In a form to update a Sensor, I have something like the following <%= sensor_form.fields_for :watches do |watches_form| %> <%= watches_form.label :label %><br /> <%= watches_form.text_field :label %> <% end %> This is indended to allow editting of the already-created Watches belonging to a Sensor. This call spits form inputs as so: <input name="sensor[watches_attributes][0][label]" ... /> <input name="sensor[watches_attributes][0][id]" ... /> When this gets submitted, the params object in the Sensor controller gets an assoc like "sensor" => { "id"=>"1", "watches_attributes"=> { "0"=>{"id" => "1", "label" => "foo"}, "1"=>{"id" => "2", "label" => "bar"} } } For a has_many, accepts_nested_attributes_for update to work upon the @sensor.update_attributes call, it seems that that attributes key really must map to an Array. From what I've seen in the examples, the combination of has_many, accepts_nested_attributes_for, and sensor_form.fields_for should allow me to pass the resulting params object directly to @sensor.update_attributes and update each related object as intended. Instead the Sensor takes place, with no errors, but the Watch objects are not updated (since "watches_attributes" maps to a Hash instead of an Array?) Have I missed something?