Normalizing Data with Document References

Data normalization is the process of organizing documents and collections to minimize redundancy and dependency. You normalize data by identifying object properties that are subobjects and that should be stored as a separate document in another collection from the object’s document. Typically you do this for objects that have a one-to-many or many-to-many relationship with subobjects.

The advantage of normalizing data is that the database size will be smaller because only a single copy of each object will exist in its own collection instead of duplicated on multiple objects in a single collection. Also, if you modify the information in the subobject frequently, you only need to modify a single instance rather than every record in the object’s collection that has that subobject.

A major disadvantage of normalizing data is that when you look up user objects that require the normalized subobject, a separate lookup must occur to link the subobject. This can result in a significant performance hit if you are accessing the user data frequently.

An example of when it makes sense to normalize data is a system that contains users that have a favorite store. Each User is an object with name, phone, and favoriteStore properties. The favoriteStore property is also a subobject that contains name, street, city, and zip properties.

However, thousands of users may have the same favorite store, so there is a high one-to-many relationship there. Therefore, it doesn’t make sense to store the FavoriteStore object data in each User object because that would result in thousands of duplications. Instead, the FavoriteStore object should include an _id object property that can be referenced from documents in the user’s FavoriteStores collection. The application can then use the reference ID favoriteStore to link data from the Users collection to FavoriteStore documents in the FavoriteStores collection.

Figure 11.1 illustrates the structure of the Users and FavoriteStores collections described above.

Figure 11.1 Defining normalized MongoDB documents by adding a reference to documents in another collection.