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?

score 8 · Accepted Answer · answered Nov 30 '11 at 18:01

8

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.

answered Nov 30 '11 at 18:01

Ian Mercer

38,490
8
97
133

Thanks. What is the document size limit? – Randomblue Nov 30 '11 at 18:24
@randomBlue document size: http://stackoverflow.com/questions/4667597/understanding-mongodb-bson-document-size-limit – Petrogad Nov 30 '11 at 19:02
1

Perhaps better to refer to the source: 16MB currently: http://www.mongodb.org/display/DOCS/Documents – Ian Mercer Nov 30 '11 at 19:09

score 1 · Answer 2 · edited Dec 01 '11 at 17:35

1

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.

edited Dec 01 '11 at 17:35

David O.

366
1
7

answered Nov 30 '11 at 17:57

Tomasz Nurkiewicz

334,321
69
703
674

score 0 · Answer 3 · answered Nov 30 '11 at 18:04

collections is a container for documents. so when you say a document that contain other documents that s kinda wrong because, already, a document can have inner documents.

Collection is the unit where you put together the documents. Be aware that due to schema free design, you can put anything in a collection but it s not a good design. so collection is kinda logical container for documents. same as tables in relational world.

Why does MongoDB have collections

3 Answers3