How to create a collection in a document in MongoDB? - mongodb

Question

I am using MongoDB for the first time, and I have some experience with NoSQL databases.
I am attempting to replicate behaviour that I have managed to achieve on Google's Cloud Firestore:
I want to create a collection within a document. I have not been able to replicate this behaviour using MongoDB as I cannot find code in the documentation. Is this behaviour even possible please?
Thanks in advance.
Edit:
Here is a screenshot of a sample document in biometric_data :

Answer 1

MongoDB has embedded documents which can be used to store the same data. You can try creating an array of sub-documents (each having name and data property):
{
name: "",
email: "",
...otherFields,
biometric_data: [
{
name: "glucose",
data: {
preferred_unit: "mg/dL"
// Add new properties as required
}
},
{
name: "weight",
data: {
preferred_unit: "KG"
}
}
],
...templateData
}
However, a document's size in MongoDB cannot exceed 16 MB. If number of fields in biometric_data are limited then you can use sub-documents otherwise you might have to create another collection to store those as documents (generally preferred for chat apps or where number of sub-documents can be really high).
Sub-collections (in Firestore) allow you to structure data hierarchically, making data easier to access. For example, users and posts collections can be structured in either of the ways below:
With sub-collection
users -> {userId} -> posts -> {postId}
Root level collections
users -> {userId}
posts -> {postId}
Though if you use root level collections, you must add a userId in posts document to identify who the owner of a post is.
If you use nested documents way in MongoDB, you are likely to hit the 16 MB document limit if any of the users decides to add many posts. Similarly if the biometric_data array can have many documents, it'll be best to create another collection.
Firestore's sub-collections and documents do not count towards 1 MB max doc size of parent document but nested documents in MongoDB do.
Also checkout:
Firestore - proper NoSQL structure for user-specific data
Is mongodb sub documents equivalent to Firestore subcollections?