Ember relationships vs mongodb

i'm trying to create a belongsTo relationship. When some user create a booking, i'm receiving this in the server:

{ data: 
   { attributes: 
      { participants: '10',
        email: 'Marcelo',
        name: 'nome',
        observation: 'obs',
        phone: '120829289' },
     relationships: { event: [Object] },
     type: 'bookings' } }

Is this correct? And if it is.. what should i register in my mongodb model? A object or string?

I was trying to register events: type: String, and i also try relationships: type: Object.