Relation

There are two ways to present the relation.

There is no strict rule to tell which method to use. It depends on the use cases.

• Embedded document

  It's easy to read and write, but the size of a document can be huge.

  Attention: the max size of a document is 16 MB.

  It may waste the space of storage if there are duplication, and waste the bandwidth if we fetch the related collections which aren't uses.

• references

  use ObjectId.

  It's used when you want to fetch the data of a collection but don't want the data of its related collections.

  It can reduce the size of a document and save the bandwidth. However, you need multiple requests if you need the related collections.

One to One

For example, a patient has one disease summary.

Usually, we use Embedded document if there is no reason to split them.

One to Many

For example, a question has many answers.

Usually, we use Embedded document if there is no reason to split them.

Many to Many

An author has many books, and a book has many authors.

Usually, we use reference to split them.