Using addToSet inside an array with MongoDB

That worked perfectly, thanks! Can you expound on the $ positional character? Is it because you don't know which line # (per se) is walkinSteps in the brackets... within the _id, date, goalStatus...?

Also what are your suggestions on a separate history collection? Reason being? What am I doing wrong?

Exactly. "The positional $ operator, when used with the update() method acts as a placeholder for the first match of the update query selector". I'll edit my answer to expand on the separate collection.

Deleting and then inserting documents that do not fit into those free extents will cause fragmentation but updating will merely reallocate the extent in which the document resides (or should, there have been a few bugs but I think they are fixed). I am unsure about the lack of size shrink on a document if you shrink it. The aggregation framework could easily make the querying of data in this array really fast

The freed up space that a moved doc leaves behind only allows for a properly sized document though? With many users with different number of history days that might be problematic. Doing a repair on the DB is afaik the only way to compact documents that have shrunk.

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Sammaye · Accepted Answer · 2013-01-06 19:36:15Z

1

Even though @Justin Case puts the right answer he doesn't explain a few things in it extremely well.

You will notice first of all that he gets rid of the resolution on dates and moves their format to merely the date instead of date and time like so:

day = {_id: ObjectId(), date: ISODate("2013-01-07"), walkingSteps:0, goalStatus: 0}

This means that all your dates will have 00:00:00 for their time instead of the exact time you are using atm. This increases the ease of querying per day so you can do something like:

db.col.update(
    {"_id": ObjectId("50db246ce4b0fe4923f08e48"), 
    "history.date": ISODate("2013-01-07")}, 
    {$inc: {"history.$.walkingSteps":0}}
)

and other similar queries.

This also makes $addToSet actually enforce its rules, however since the data in this sub document could change, i.e. walkingSteps will increment $addToSet will not work well here anyway.

This is something I would change from the ticked answer. I would probably use $push or something else instead since $addToSet is heavier and won't really do anything useful here.

The reason for a separate history collection in my view would be what you said earlier with:

Yes, the amount of history items for that day.

So this array contains a set of days, which is fine but it sounds like the figure that you wish to get walkingSteps from, a set of history items, should be in another collection and you set walkingSteps according to the count of the amount of items in that other collection for today:

db.history_items.find({date: ISODate("2013-01-07")}).count();

answered Jan 6, 2013 at 19:36

Sammaye

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4 Comments

$push will add duplicate entries for the same day though which isn't necessarily what OP wants. If the add-day query will never be called twice then I agree that $push is faster.

@JustinCase If he increments or changes walkingSteps at all which he intends to do then $addToSet will readd that element too, $addToSet evaluates the entire array element being added, he probably should be $pushing where the day does not already exist.

I'm trying to automate adding a day, say at midnight so it'll only need to happen once for the given day. Then updating "walkingSteps" and "goalStatus" values during that day.

@user191277 $push will do just fine here since you are only running the query once (at midnight).

coderLMN · Accepted Answer · 2013-01-06 19:04:07Z

0

Referring to MongoDB Manual, $ is the positional operator which identifies an element in an array field to update without explicitly specifying the position of the element in the array. The positional $ operator, when used with the update() method and acts as a placeholder for the first match of the update query selector.

So, if you issue a command to update your collection like this:

db.history.update(
   {someCriterion: someValue },
   { $push: { "history": 
               {"_id": {
                   "$oid": "50db2316e4b0fe4923f08e12"
               },
              "date": {
                   "$date": "2012-12-24T15:26:15.321Z"
               },
           "walkingSteps": 10,
           "goalStatus": 1
           } 
   } 
)

Mongodb might try to identify $oid and $date as some positional parameters. $ also is part of the atomic operators like $set and $push. So, it is better to avoid use this special character in Mongodb.

answered Jan 6, 2013 at 19:04

coderLMN

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5 Comments

Ok. I may slowly be catching on. So if just using $ to target an element isn't the best, how would you convert this db.users.update(query, {$set: {"history.$.walkingSteps": 6}}) to be handled better?

@user191277 Hmm not sure if what above makes sense, the $date and $oid are BSON types and handled by MongoDB properly, the positional operator will work just fine here.