Querying for array in embedded list

Question

Supposed I have a mongo document that looks similar to the following:

{
   'foo':1
   'listOfLists' : [ [1,2],[3,4] ]
}

(Yes I am aware this isn't how it "really" looks but it should be simple enough for explanation purposes.)

If I wanted to write a query that would check to see if the listsOfLists list object contains the combination of [3,4], how could I go about doing that?

Could I do something like

collection.find({'listsOfLists' : {'$elemMatch' : [3,4] } })

Neil Lunn · Accepted Answer · 2016-04-06 01:36:31Z

1

collection.find({ 'listsOfLists': [3,4] }).

It's just a "direct match" on the property. MongoDB will look at each array element automatically. You don't need $elemMatch here.

If you were to use it, you need an operator expression, such as $eq:

collection.find({ 'listsOfLists': { '$elemMatch': { '$eq': [3,4] } } }).

But that of course is not required unless there are "two or more" conditions that actually need to match on the array elements. Which is what $elemMatch is actually for.

answered Apr 6, 2016 at 1:36

Neil Lunn

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2 Comments

K.Niemczyk Over a year ago

So, in a related item. Could I use $elemMatch to check to see if just the First element in a list matches a query? Something like { '$elemMatch' : { '$eq' : 3} }

Neil Lunn Over a year ago

@K.Niemczyk That would actually be Another Question to Ask, but it is one with a logical answer. Post another question here if you cannot work it out.

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