Question

When I call ensureIndex from the mongo shell on a collection for a compound index an _id field of type ObjectId is auto-generated in the index object.

> db.system.indexes.find();
{ "name" : "_id_", "ns" : "database.coll", "key" : { "_id" : 1 } }
{ "_id" : ObjectId("4ea78d66413e9b6a64c3e941"), "ns" : "database.coll", "key" : { "a.b" : 1, "a.c" : 1 }, "name" : "a.b_1_a.c_1" }

This makes intuitive sense as all documents in a collection need an _id field (even system.indexes, right?), but when I check the indexes generated by morphia’s ensureIndex call for the same collection *there is no _id property*.

Looking at morphia’s source code, it’s clear that it’s calling the same code that the shell uses, but for some reason (whether it’s the fact that I’m creating a compound index or indexing an Embedded document or both) they produce different results. Can anyone explain this behavior to me?

Answer 1

Not exactly sure how you managed to get an _id field in the indexes collection but both shell and Morphia originated ensureIndex calls for compound indexes do not put an _id field in the index object :

> db.test.ensureIndex({'a.b':1, 'a.c':1})
> db.system.indexes.find({})
{ "v" : 1, "key" : { "_id" : 1 }, "ns" : "test.test", "name" : "_id_" }
{ "v" : 1, "key" : { "a.b" : 1, "a.c" : 1 }, "ns" : "test.test", "name" : "a.b_1_a.c_1" }
>

Upgrade to 2.x if you’re running an older version to avoid running into now resolved issues. And judging from your output you are running 1.8 or earlier.