Bookshelf.js - hasOne relationship not working well

I'm fairly new to bookshelf.js so please bear with me.

Since I'm learning, I'm using MySQL and have a really simple structure. Namely, two entities, with one secondary key. Here's my structure:

CREATE TABLE IF NOT EXISTS `city` (
  `city_id` int(11) NOT NULL,
  `code` int(11) NOT NULL,
  `name` varchar(255) COLLATE utf8_bin NOT NULL
) ENGINE=InnoDB DEFAULT CHARSET=utf8 COLLATE=utf8_bin;

CREATE TABLE IF NOT EXISTS `people` (
  `people_id` int(11) NOT NULL,
  `city_id` int(11) NOT NULL,
  `name` varchar(255) COLLATE utf8_bin NOT NULL,
  `age` int(11) NOT NULL
) ENGINE=InnoDB DEFAULT CHARSET=utf8 COLLATE=utf8_bin;

ALTER TABLE `city`
  ADD PRIMARY KEY (`city_id`);

ALTER TABLE `people`
  ADD PRIMARY KEY (`people_id`), ADD KEY `city_id` (`city_id`);

ALTER TABLE `people`
ADD CONSTRAINT `people_ibfk_1` FOREIGN KEY (`city_id`) REFERENCES `city` (`city_id`);

And here's my code in expressjs, using bookshelf:

var City = bookshelf.Model.extend({
    tableName: 'city'
});

var User = bookshelf.Model.extend({
  tableName: 'people',
  city: function(){
    return this.hasOne(City);
  }
});

This is a sample code of how I run my query:

new User({person_id: req.params.id })
.city()
.fetch()
.then(function(user){
    res.send(user.toJSON());
})
.catch(function(error){
    res.send(error);
});

However, when I run the application, I get the following error

{"code":"ER_BAD_FIELD_ERROR","errno":1054,"sqlState":"42S22","index":0}

I've enabled the debug mode in bookshelf, and this is the query it tries to execute:

{ __cid: '__cid2',
  method: 'select',
  options: undefined,
  bindings: [ undefined, 1 ],
  sql: 'select `city`.* from `city` where `city`.`person_id` = ? limit ?' }

Now as I've said it, I'm new to bookshelf. But this makes no sense; why does it select "person_id", instead of "city_id"?

Thank you for your time and answers!