Why does apply_filters behave different inside and outside a loop?

Question

What I want: the array of posts returned by creating $query = new WP_Query($args);

Why: to return specific content as a sort of API request in json format, ready to display on another site

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What I tried first:

foreach($query->posts as $post) {
    $post->post_content = apply_filters('the_content', $post->post_content);
};

This performed the autop and do_shortcode filters correctly, but no the oembed transformation.

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What I ended up doing:

while($query->have_posts()) : $query->the_post();
    $query->posts[$query->current_post]->post_content = apply_filters('the_content', get_the_content());
endwhile;

The only difference in output, is that inside the loop it performs the oembed, outside it does not. Where does this difference come from, and is there a better way I should have done this?

I checked global $wp_filters and oembed is definitely listed under [8], so it wasn't a filter loading issue.

Answer 1

There is a difference because you are not using the_post function in the first example. What this function does is it calls setup_postdata function, which sets up all the globals needed for other functions to work. You can call it manually, just at the beginning of your foreach loop, like this: setup_postdata( $post ). In addition it is possible that you would also need to call a global $post; before your foreach loop.