Question

What I am trying to do is add a new {field:value} for a blog post. So for example, if I wanted to start tracking impressions on websites.blog_posts.url: ‘http://www.example.com/01.html’ how can I add that impressions attribute for that blog post?

My current document structure:

{ 
email_address: '[e-mail]', 
password: 'random_password',
first_name: 'John',
last_name: 'Doe',
user_type: 'WEBMASTER',
newsletter: 'NO',
websites: [{
    main_title: 'My Blog Website',
    main_url: 'http://www.example.com',
        blog_posts: [{
            url: 'http://www.example.com/01.html',
            title:'My first blog post',
            description: 'My first description.'
        }, { 
            url: 'http://www.example.com/02.html',
            title: 'My second blog post',
            description: 'My second description.'
        }, { 
            url: 'http://www.example.com/03.html',
            title: 'My third blog post',
            description: 'My third description.'
        }, { 
            url: 'http://www.example.com/04.html',
            title: 'My fourth blog post',
            description: 'My fourth description.'
        }]
    }]
}

Here is what I thought would work using update and making upsert TRUE.

db.my_collection.update( {‘websites.blog_posts.url’: ‘http://www.example.com/01.html’ }, {‘$set’: {‘websites.blog_posts.impressions’: 549}}, true )

The error that I received is:
*can’t append to array using string field name [blog_posts]*

Maybe “$set” is not correct for this or maybe I can not reference that deep with dot notation? I just started using MongoDB yesterday, any help would be great.

Thank you!

Answer 1

What you’re trying to do is not possible given your schema. Dot-notation can be multi level but if there’s more than one level that is an array it can no longer be addressed using the positional operator ‘$’.

E.g. you’d need to do :

db.my_collection.update( 
    {'websites.blog_posts.url': 'http://www.example.com/01.html' },
    {'$set': {'websites.$.blog_posts.$.impressions': 549}},
     true );

But having two position operators in the update is not possible since MongoDB can only determine the position of an element in the first array.

Your only option is to redesign your schema to have a dedicated collection of user websites (which is better for other reasons too in this case).