Why does MongoDB have collections

Question

MongoDB being document-oriented, the structure of collections seems to be a special case of documents. By that I mean one can define a document to contain other documents. So a collection is just a document containing other documents.

So why do we need collections after all?

Answer 1

Logically yes, you could design a database system like that, but practically speaking no.

• A collection has indexes on the documents in it.
• A collection requires the documents in it to have unique ids.
• A document is limited in size.

Answer 2

• Object ids (_id top-level document attribute) must be unique within a collection. Multiple collections may have the same _id, just like in RDBMs where the key constraint is per-table, yet multiple tables may contain the same value for a key.