pre_get_posts and the blog page

Question

I am trying to set up my new theme in a proper way. I have created a custom home page for my new theme called "template-home.php" and then I created a "blog" page. I went to settings and changed my "Reading" settings to "A static page" and I set my front page to my home template and then I set my "Posts page" to "blog". I did not assign a custom template to that page. It is my understanding that the index.php file would be used to get those posts. I have been trying to use is_page() and is_category() to say that if it's the blog page, then get the blog posts. My hope was that when turning on the theme for the first time, all posts would be displayed on the front page but if I visited the blog page, the conditionals would kick in and get only the posts on the blog page. I thought I might be able to accomplish this task by using this code

add_action( 'pre_get_posts', 'blog_query');

function blog_query( $query ){
    if ( $query->is_page('blog') ) {
    $query->set( 'category_name', 'blog' );
   }    
}

That isn't working. That gives me an error on the front page "Trying to get property of non object" I am just not sure which method to use to set up the blog page. Should I set a custom template for it and let the index page get all posts or try using the pre_get_posts?

Your input is appreciated.

Answer 1

Your blog page is actually your home page, so you need to use is_home(). For the static frontpage, is_front_page() would be used

EDIT

Sorry for being confusing :-). This how it works when a static frontpage is set

The page set as your frontpage is actaully just a cover page. When you think of a book, this is the page that will tell the reader what the book is all about. The page used here will be the custom page template set by you. The correct naming should be front-page.php, but any page template will do. The proper conditional tag to use here will be is_front_page()

Now, the blogpage, whether explicitely set or not, will either use home.php, and if that is not available, will use index.php. Not many people knows this, but this is regarded as your site's homepage, the normal index page you will see when a static frontpage is not set. To target your blogpage, you'll need to use is_home() conditional check. Strange, but true

To restrict your blogpage to one category only, use pre_get_posts as you did, just change your conditional tag accordingle

function only_one_cat_blogpage($query) {
    if( !is_admin() && $query->is_home() && $query->is_main_query() ){
        $query->set( 'cat', 'ID OF CATEGORY' );
    }
}
add_action( 'pre_get_posts', 'only_one_cat_blogpage' );

EDIT 2

For further reading, check out

• Creating a Static Front Page

Answer 2

Try this. As mentioned by Pieter you will have to check for is_home() and set conditions accordingly.

function show_category_posts( $query ) {
    if ( $query->is_home() && $query->is_main_query() ) {
        $query->set( 'category_name', 'blog' );
    }
}
add_action( 'pre_get_posts', 'show_category_posts' );