Java Spring, @RequestBody - how to receive valid JSON with two connected entities

I've got simple

DB SCHEMA

Contact
-------
int: id
varchar: name

Note
-------
int: id
int: id_contact
varchar: title

As you can see, tables are connected. My goal is to add note by AJAX request with id_contact provided.

FORM should look like that

<form action="add-note" id="add-note">
    <input type="hidden" name="id_contact" value="20" /> <!-- just a dummy value which corresponds with contact record in DB -->

    note: 
    <input type="text" name="title" class="title">

    <input type="submit" />
</form>

Method to process AJAX request

@RequestMapping(value = "/add-note", method = RequestMethod.POST)
public ResponseEntity<Note> addNote(@RequestBody Note note) {

    // do something with note...

    return new ResponseEntity<Note>(note, HttpStatus.OK);
}

If I don't try to provide id_contact, method addNote works fine, but I'm not able to make spring to convert JSON to Note with Contact in it.

Does anyone know how to achieve it? Thanks in advance