symfony2 php访问另一个实体的对象

I know its very lame question to ask. I have One Entity ContentSelector and another is Content in Content Selector I have a property Content where I have Id. I am trying to access the Content propety but always getting error. This is what I am trying to do.

$cs = $pages->getContentSelector();

it returns:

[

{
    "id": ​22,
    "title": "my Title",
    "content": "11021,10930,10874,10766,10954,10705,10948,10934,11048,11049,10831"
}

]

now I want to get the content property like $cs->content getting error like:

"Notice: Undefined property: Doctrine\\ORM\\PersistentCollection::$content"

tried with getContent(); getting error that the method doesnt exist. but in ContentSelector the method actually exists.

how can I easily get the content ids?