Content won't load when using action the_content

Question

When I use add_action( 'the_content'...

The content of the page just won't load. Also the content I am trying appears in all kinds of places in the page. Why?

Is there a way to force it to only appear under my post?

function __construct()
{
    add_action( 'wp_footer', array(&$this, 'loadRatingsJs') );
    add_action( 'wp_ajax_valnetincGetRatings', array(&$this, 'getRatings' ) );
    add_action( 'wp_ajax_nopriv_valnetincGetRatings', array(&$this, 'getRatings' ) );
    //add_action( 'the_excerpt', array(&$this, 'getRatings' ) );
    add_filter( 'the_content', array(&$this, 'getRatings' ) );
}

public function getRatings($content)
{
    //echo $postID;
    if(function_exists('the_ratings')) {
        /*if(is_null($postID))
        {
            the_ratings();
        } else {
            the_ratings('div', $postID);
        }*/
        $rating = the_ratings();
        return $content.$rating;
    }
}

Answer 1

the_content is not an action, it is a filter. You should be concatenating a string and returning it. It sounds like you are echoing content directly.

That is:

// wrong
function test_content($content) {
  echo 'this is wrong';
}
add_filter('the_content','test_content');

// right
function test_content($content) {
  return 'this is right '.$content;
}
add_filter('the_content','test_content');

Answer 2

One potential problem is that you don't return anything if your conditional fails:

public function getRatings($content)
{
    //echo $postID;
    if(function_exists('the_ratings')) {
        /*if(is_null($postID))
        {
            the_ratings();
        } else {
            the_ratings('div', $postID);
        }*/
        $rating = the_ratings();
        return $content.$rating;
    }
}

To simplify:

public function getRatings($content)
{
    // Conditional
    if(function_exists('the_ratings')) {
        // Do stuff
        // Return modified content
        return $content.$rating;
    }
}

The problem is: you don't return anything if the_ratings() function doesn't exist. You need to account for that condition:

public function getRatings($content)
{
    // Conditional
    if(function_exists('the_ratings')) {
        // Do stuff
        // Return modified content
        return $content.$rating;
    }
    // Condition is false;
    // Just return the original content
    return $content;
}

An even simpler way (perhaps) is to return outside the conditional, and only modify inside the conditional:

public function getRatings($content)
{
    // Conditional
    if(function_exists('the_ratings')) {
        // Do stuff
        // Modify content
        $content .= $rating;
    }
    // Return content, modified or unmodified
    return $content;
}