MongoDB: Unique and sparse compound indexes with sparse values
mongodb
Solution
Keep in mind that mongodb can only use one index per query (it won't join indexes together to make a query on two fields that have separate indexes faster).
That said, if you want to try to check for uniqueness, you could do a query from the app before inserting (which only partially solves the problem, because there's a gap between when you query and when you insert).
You might want to vote on this JIRA issue for filtered indexes, which will probably help your use case: https://jira.mongodb.org/browse/SERVER-785
Problem
I'm trying to store the following link: ``` URL = { hostname: 'i.imgur.com', webid: 'qkELz.jpg' } ``` I want a unique and sparse compound index on these two fields because: - A combination of `hostname` and `webid` should be unique. - `webid` will always be queried with `hostname`. - `webid` need not be globally unique. - A `URL` need not have a webid. However, when I do this, I get the following error: ``` MongoError: E11000 duplicate key error index: db.urls.$hostname_1_webid_1 dup key: { : "imgur.com", : null } ``` I guess in the case of compound indexes, nulls are counted, whereas in regular indexes, they are not. Any way out of this problem? For now I'm just going to index `hostname` and `webid` separately.