#mongo arrays

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MongoDB: Filtering an Array

Adding to this old post on querying arrays and building on the $ operators lesson from the M102 course on the 10gen website, let's see how to get only the interesting elements of an array.

Let's say we have a coders collection. A generic document looks like this:

{ id: "0001", name: "DeK", gender: "M", country: "Italy", codesIn: "Java" friendWith: [ {name: "Dude", codesIn: "PHP", gender: "M"}, {name: "Dork", codesIn: ["Haskell", "Perl"], gender: "M"}, {name: "Derp", codesIn: "JavaScript"} {name: "Derpina", codesIn: "Perl", gender: "F"} ] }

To match multiple values on multiple elements of an array, the $elemMatch operator is provided:

db.coders.find(friendWith: {$elemMatch: {codesIn: "Perl", gender: "F"}})

will return an array containing only Derpina, that is the element that both codes in Perl and is a female. For an or match, that is to get elements that are either Perl coders or females, standard dot notation will just do:

db.coders.find({friendWith.codesIn: "Perl", friendWith.gender: "F"})

will return an array with Dork and Derpina. Finally, a query on a subdocument:

db.coders.find({friendWith: {codesIn: "Perl", gender: "F"}})

will look for friends with exactly this structure: code in Perl, gender is female and have no name. In our example, DeK has no friends with this features.

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Updating an array in a MongoDB document

Using the $set operator on an array value will replicate the "wholesale" issue on a smaller scale:

db.people.update({name: "Smith"}, {$set: {"friends": "Girl"}})

the whole array is replaced by the new value. If we want to add a value, the $push operator will do:

db.people.update({name: "Smith"}, {$push: {"friends": "Girl"}})

To avoid duplication, the $addToSet will consider the array as a set and return it untouched if the value is already present:

db.people.update({name: "Smith"}, {$addToSet: {"friends": "Girl"}})

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Querying arrays in Mongo DB

When looking into fields that are (or can possibly be) arrays, Mongo DB will smartly behave polymorphically. The query:

db.messages.find({ "headers.To": "[email protected]" }, {"headers": 1}).limit(10).pretty()

will also include the result of the where query, even though the To field of that mail is an array: in that case, Mongo inspects each value of the array and verifies if at least one matches our argument.

To look for emails sent to both this and that recipient, use the $all operator:

db.messages.find({ "headers.To": {$all: ["[email protected]", "[email protected]" ]} }, {"headers": 1}).limit(5).pretty()

To look for emails sent to at least one but not necessaraly all of the specified recipients, use the $in operator:

db.messages.find({ "headers.To": {$in: ["[email protected]", "[email protected]" ]} }, {"headers": 1}).limit(5).pretty()

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Looking for multivalue fields in MongoDB

I was doing the final exam of 10gen M101P course (Mongo for Python developers) and wanted to find in the given dataset one of the emails having more than one recipient.

db.messages.find({ $where: "if (this.headers['To'] && this.headers['To'].length >= 2) return this" }).limit(1).pretty()

This will return the whole document. To select only some fields, projection is the way to go.

The limit is basically because using JavaScript is inherently slower than using the normal query functions.

Additional links:

Stack Overflow thread

Mongo DB Manual: $where