Comparing arrays with meta_query in pre_get_posts

I’m working on a form for filtering a list of posts using custom fields created with the Advanced Custom Fields plugin. Because of the form, I’m using the pre_get_posts action to change the query via GET requests. (following code references are either PHP or dumped from print_r())

I set the meta_query like this:

$query->set('meta_query',$filter);

and $filter looks like this:

Array
(
    [0] => Array
        (
            [key] => delivery_method
            [value] => Array
                (
                    [0] => Online
                    [1] => Scheduled

                )

            [compare] => IN
        )
)

The custom field that I am querying is structured like this:

Array (
    [delivery_method] => Array (
        [0] => Online
        [1] => Scheduled
    )
)

When I look for posts with [compare] => IN (as above), no posts are returned. When I look for posts with [compare] => NOT IN, all of the posts are returned.

I am trying to return only those posts which have a specific “delivery method”. Is there a way to compare the two arrays that I missed? or do I have to somehow explode one of the arrays and compare individual values against an array?

The ACF Documentation recommends checking the values individually rather than simultaneously using an array.

The following code is from the ACF Documentation for the Checkbox field type:

http://www.advancedcustomfields.com/resources/field-types/checkbox/

/*
*  Query posts for a checkbox value.
*  This method uses the meta_query LIKE to match the string "red" to the database value a:2:{i:0;s:3:"red";i:1;s:4:"blue";} (serialized array)
*  The above value suggests that the user selected "red" and "blue" from the checkbox choices
*/

$posts = get_posts(array(
    'meta_query' => array(
        array(
            'key' => 'field_name', // name of custom field
            'value' => '"red"', // matches exaclty "red", not just red. This prevents a match for "acquired"
            'compare' => 'LIKE'
        )
    )
));

Therefore the query in pre_get_posts should look like this:

$filter = array(
    array(
        'key' => 'delivery_method'
        'value' => '"Online"'
        'compare' => 'LIKE'
    ),
    array(
        'key' => 'delivery_method'
        'value' => '"Scheduled"'
        'compare' => 'LIKE'
    )
)

$query->set('meta_query',$filter);