Collections and Embedded Documents in MongoDB

October 22, 2013 — Chris Foster

When someone is approaching MongoDB from the SQL world, a very common confusion regarding database structure is when to use embedded documents, and when to create an entirely new collection. This distinction is very important because, although MongoDB is schemaless in nature, whether or not an element of your database is structured as embedded documents or a separate collection will change your code a fair amount. Making this change later on can represent a fair amount of work, so it helps to get this right the first time.

There is no "right answer" to this question, as it depends entirely on the situation at hand. The natural tendency of people coming from the SQL world is to stick everything in separate collections, but often this is very unnecessary and will cause serious performance impacts. However, mistakenly placing something within another document may lead to pain further down the road.

A set of rules I have found useful is to ask yourself the following questions:

  1. Does the embedded document relate to one or more other collections?
  2. Will you most often need the embedded document without the parent document?
  3. Will you most often need the parent document without the embedded document?

If the answer to two or more of these is yes, you likely will want a separate collection. If the answer to only one of these is yes, a separate collection should still be considered, but likely not needed.

Examples

Comments on a blog

You would like to create a system where people may submit comments on blog posts. The problem is that you are unsure if you should store the comment on the post document, or create a separate collection named comments.

Does the embedded document relate to one or more other collections?

No. A comment is typically related to only the post that it is commented on. There may be some situations where this is not true, such as if you provided comment author accounts for editing. However, even this is not a very convincing reason by itself to separate the comment into a separate collection.

Will you most often need the embedded document without the parent document?

Again, the answer is no. You likely will not often need to load a comment without also needing the context of the post.

Will you most often need the parent document without the embedded document?

In the majority of cases, the answer here is no. Most of the time you use this object, someone will be viewing a blog entry. You will want to both display the post and the comments at once, so it makes sense to fetch those together.

Overall, comments for a blog is a very good candidate for embedded documents.

Students in a class

You have a school management system, and you would like to enable students to enrol in a particular class. You are unsure if you should store the student objects on the class, or create a separate collection named students.

Does the embedded document relate to one or more other collections?

Typically, we can assume yes. A student will likely relate to other things, such as an assignment or school object. Also, a very important note is that each embedded document will likely relate to multiple documents in the classes collection, which is a very strong hint you need a separate collection.

Will you most often need the embedded document without the parent document?

The answer here will often be yes. If you want any sort of student information panel or want to have students enrolled in different classes, then you will often want the student document without needing the context of each class.

Will you most often need the parent document without the embedded document?

Probably no for this one. It depends on what operation we are doing most often with the class, but I imagine that when we fetch a class we would likely need at least one student as well.

Overall, students in a class are probably better suited for a separate collection. It's important to keep in mind the scope of the problem you are solving with the data, and the operations that will be done most commonly. That said, a student is a very relational piece of data and better fits a separate collection.

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