Your code makes Rails do object_a.things and put the result in the text area. when outputting an object like a string, ruby prints out what you see, class name and the (internal ruby) object's id: #<Thing: 0x460efb0>
you will have to do something like this: <%= text_area_tag "things", @object_a.things.map{ |t| t.name }.join(" ") %>
then, when submitting the form, the content will be in params[:things] if however you want it in params[:object_a][:things], change it like this:
<%= text_area_tag "object_a[things]", @object_a.things.map{ |t| t.name }.join(" ") %>
be aware that this could lead to problems when doing mass assignment: @object_@ =Object_a.new(params[:object] => "things" is no attribute of object_a (it's only an associtiaon), and produce an error. you can however catch this with a virtual attribute...