如何限制Sonata多对多字段中的可用选项？

I am working on a program built with the Sonata Admin bundle. I have a ScheduleAdmin class that includes the following logic:

    $formMapper
        ->add(
            'markets',
            null,
            [
                'required' => true,
                'label'    => 'shared_countries',
                'data'     => $this->getMarketsByUser($this->getUser())
            ]
        )
    ;

The relationship between Schedules and Markets is a many-to-many relationship.

The getMarketsByUser() method always returns one market -- USA for example. What I get in my browser is a field that includes my market, but also includes an autofill dropdown containing every other country.

Now I want to get rid of those other autofill options, allowing only the market affiliated with my user. How do I do that?

Well that was simple. I took a look at this page and then changed my add() method call to look like this:

    $formMapper
        ->add(
            'markets',
            null,
            [
                'required' => true,
                'label'    => 'shared_countries',
                'data'     => $this->getMarketsByUser($this->getUser()),
                'choices'     => $this->getMarketsByUser($this->getUser()),
            ]
        )
    ;

... and now it works swell.