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Pre-Note: This is aimed at the JavaScript SDK of Parse, however it may help if you are using a different SDK like Android. So if you are having issues with Relations/Pointers give this a read!
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Edit: @tyaramis sparked a great question of how to do multi-dimensional querying in Parse, so I made another protip dedicated to his question: https://coderwall.com/p/-uzbrq/multi-dimensional-querying-parse-com-javascript-sdk

When I first started to use Parse I was tasked with creating a database for our first rendition of our mobile application for DD buddy. We knew what information we needed/wanted at the time, the trick was to set up an efficient database that didn't repeat information. Coming from a MySQL background and knowing nothing other than PHPMyAdmin, I was fully aware of relationships between database tables, or in Parse's case, Classes. However, I did not understand how I would set up a relationship between classes in Parse.

When I started to dive into Parse's JS SDK Documentation I found the sections on Relationships. The documentation does a great job on describing the situations you would want to use a Pointer, an Array (of either Pointers or Relations), a Relation, or a Join Table. What it lacks though is a specific FULL example of how to query these pointers in order to receive useful information. It is actually quite simple, and you will realize the enormous benefits of using pointers/relations by the time this article is done!

Before I start, please do not take any of this as me saying the documentation Parse supplies is lacking. If you dive into their specific SDK documentation it all is laid out in a very easy to understand manner, and some of it just takes time and messing around with to understand. This just happens to be one sections that I had issues implementing due to lack of information.

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Class Structures

First let's set up three Classes in Parse. Users, Posts and Comments.

_User Structure:

• objectId
• username
• password
• name - string

Posts Structure:

• objectId
• postName - string
• postMsg - string
• postAuthor - Pointer< _User > (Pointer to the User Class)

Comments Structure:

• objectId
• msg - string
• post - Pointer< Posts > (Pointer to the Posts Class)
• user - Pointer< _User > (Pointer to User Class)

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Now, let's get all the posts in the Posts class and find out the name of the user that is its author.

var Posts = Parse.Object.extend("Posts");
var query = new Parse.Query(Posts);

What we just did is set up a query for our JavaScript code to talk to our Posts class.

query.include("postAuthor");

This line of code is what allows us to get the information from the user entry to postAuthor is "pointing" to without doing another query.

query.find().then(function(results){
    /* Go Through Each Post */
    var postsArray = new Array();
    for(i in results){
        /* Set obj to current post*/
        var obj = results[i];
        /* Get Post's Name */
        var postName = obj.get("postName");
        /* Get Post's Message */
        var postMsg = obj.get("postMsg");
        /* Get Post Author's Name */
        var authorName = obj.get("postAuthor").get("name");

        /* Let's Put the Post Information in an Array as an Object*/
        postsArray.push({
            post:{
                name: postName,
                msg: postMsg
            },
            author: authorName
        });
    }
})

Then all we did there was, using the query we created, gather all of the posts and put their info in an object inside the postsArray.

However, if you look at where we declared authorName you can see how we got the user's/author's name. When you use the .include() method on your query you can then use .get() on the column you included, and then another .get() to get the column you wish of the class the pointer is pointing to. (E.g. postAuthor is the column we included and name is the column we want on the User class. So we can do, obj.get("postAuthor").get("name") and we'll receive back the name of the user.)

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Multi-Dimensional Includes

In some cases you might need to go further than just including one pointer. Say you are including a pointer, and on the entry you're pointing to, it has another pointer and you need the info the second level pointer is pointing to. How do we do that? Well, let's gather all the comments in the Comments class, and then we'll try to get the post author name (not the name of who commented).

Same first steps, we'll query the Comments class this time.

var Comments= Parse.Object.extend("Comments");
var query = new Parse.Query(Comments);

Next, we'll do another include. First we'll include the post column in the Comments class, because we might want to know what the post was that this comment is on. Then, we'll do another include in order to have access to the postAuthor, we will be using dot notation in order to access multi-dimensional pointers. If we just wanted the postAuthor we could leave out the first include, but in this example let's get both.

/*Get the Post's Info*/
query.include("post");
/*Get the Post Author's Info*/
query.include("post.postAuthor");
/*Get the Comment's Author Info*/
query.include("user");

To recap the last include, we use dot notation to include the pointer that in on the postAuthor column of the Posts class that we are accessing from the post column of the Comments class. Then we just query for the information like we did on the Posts.

query.find().then(function(results){
    /* Go Through Each Comment*/
    var commentsArray = new Array();
    for(i in results){
        /* Set obj to current comment*/
        var obj = results[i];
        /* Get Post's Name */
        var postName = obj.get("post").get("postName");
        /* Get Post's Message */
        var postMsg = obj.get("post").get("postMsg");
        /* Get Post Author's Name */
        var authorName = obj.get("post").get("postAuthor").get("name");
        /* Get Comment's Message */
        var commentMsg = obj.get("msg");
        /* Get Comment's Author*/
        var commentAuthor = obj.get("user").get("name");

        /* Let's Put the Comment Information in an Array as an Object*/
        commentsArray.push({
            post:{
                name: postName,
                msg: postMsg
            },
            author: authorName,
            comment: {
                author: commentAuthor,
                msg: commentMsg
            }
        });
    }
})

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So that wraps it up, we covered both standard including for pointers and multi-dimensional includes. If you have any questions, please let me know and I'll try my best to help!

Cheers!
Zach