From MONGOID-2195, I learned that saving an embedded document directly will not #touch the parent, even if that parent includes Mongoid::Timestamps. I therefore write an #after_save callback on the embedded document to #touch the parent.

Recently, I discovered that Mongoid sometimes sets more than just the updated_at attribute when #touch is called. The failing spec in this pull request illustrates the issue.

In the spec, I add a second location to an address on a person. The queries run on MongoDB are:

update flags:0 q:

{ _id: ObjectId('502bdae523e2f62e68000001'), addresses._id: "hobrecht" }

o:{ $pushAll: { addresses.0.locations: [

{ _id: ObjectId('502bdae523e2f62e68000003'), name: "home" }

] } }

and then for #touch:

update flags:0 q:

{ _id: ObjectId('502bdae523e2f62e68000001') }

o:{ $set:

{ updated_at: new Date(1345051365963) }

, $pushAll: { addresses.0.locations: [

{ _id: ObjectId('502bdae523e2f62e68000003'), name: "home" }

] } }

As you can see, there is a $pushAll in the second query that sets updated_at.