discontiguous array index keys

[tim.d...@gmail.com]

I just discovered that I have been inserting documents that contain arrays whose elements are indexed with odd numbers only. E.g., document looks like

{_id:1,a:[100,200,300]}

but array element keys are "1","3","5".  e.g., db.find({'a:5':300}) returns the document while db.find({'a:2':300}) does not.

This is possible because the C-driver takes a key value when you append array elements to a bson object you are building.  It is supposed to be "0", "1", "2".... but I have accidentally been using "1","3","5",... without any apparently harm, unless I try to refer to a specific array element with dot notation as in above examples.  Then it does remember what index key was used on insertion, though I am not sure how to see that in the database, or what happens after updates.

I have been adding new elements with {$push:{a:{$each:[400,500]}}} where the $each array is indexed with "1" and "3".  Does that mean I get index keys [1,3,5,1,3] or maybe [1,3,5,6,8] or [1,3,5,6,7] or [0,1,2,3,4]?

For the most part, I never refer to the indices, and did not even realize the database was saving them.  I load an array, and then use the "bson_iter_next" to walk through the elements.  I think all of my updates using $push:$each or "$addToSet:$each have been working correctly.  Assuming I never need to refer to a specific element with dot notation, is there any reason I need to repair these index keys for proper array management?

[tim.d...@gmail.com]

If anyone else has made this error, it appears that:   db.collection.copyTo(new_collection) will set array element keys to "0","1","2",... in the new collection.

[Asya Kamsky]

It looks like you ran into 
https://jira.mongodb.org/browse/CDRIVER-504

It's possible for it to cause a subtle access issue - glad you found a workaround to fixing them.  

Would you mind adding a comment with your description of the problem to the Jira ticket?   It'll help prioritize it knowing that more people have run into it. 

Asya

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Asya Kamsky
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