How to check if an array field contains a unique value or another array in MongoDB?

Question

I am using mongodb now.

I have a blogpost collection, and it has a tags field which is an array, e.g.

blogpost1.tags = ['tag1', 'tag2', 'tag3', 'tag4', 'tag5']
blogpost2.tags = ['tag2', 'tag3']
blogpost3.tags = ['tag2', 'tag3', 'tag4', 'tag5']
blogpost4.tags = ['tag1', 'tag4', 'tag5']

How can I do these search

contains tag1
contains ['tag1','tag2']
contains any of ['tag3', 'tag4']

nagle · Accepted Answer · 2013-01-29 22:40:22Z

224

Try this out:

db.blogpost.find({ 'tags' : 'tag1'}); //1
db.blogpost.find({ 'tags' : { $all : [ 'tag1', 'tag2' ] }}); //2
db.blogpost.find({ 'tags' : { $in : [ 'tag3', 'tag4' ] }}); //3

edited Jan 29, 2013 at 22:40

nagle

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answered Mar 20, 2011 at 4:29

Jeff the Bear

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7 Comments

Scott Hernandez Over a year ago

This is well documented in the help: mongodb.org/display/DOCS/…

redben Over a year ago

for the $all does it mean all elements AND in the expressed order or is it just unordered ?

Zut Over a year ago

@ScottHernandez I do not see that they mention that the field you are using as your search can be an array, and how that is handled. "field : { $in : array }". What happens when you search for an array within an array of arrays? Not specified.

Hitesh Joshi Over a year ago

Is there any INDEXING we can do on array's to stop duplicate? If yes, please guide how.

Matthias B Over a year ago

@redben its unordered like written in the docs: $all operator DOCS. Just read the example part and you will see.

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kubudi · Accepted Answer · 2013-01-10 11:43:30Z

5

My experience is that for (2) the following solution is much faster than the one with "$all":

db.blogpost.find({ $and: [ {tags: 'tag1'} ,{tags: 'tag2'} ] });

but to be honest, I do not not why. I would be interested in, if anyone knows.

edited Jan 10, 2013 at 11:43

kubudi

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answered Aug 31, 2012 at 9:57

heinob

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5 Comments

isox Over a year ago

By the way, just tested it on indexed keywords list. Absolutely the same result with $and and $all

heinob Over a year ago

Maybe that changed with newer versions in the meantime.

kubudi Over a year ago

It's circumstantial. For the "$and", mongodb making a logical "and" operation. Therefore if the first expression is false, the second are not taken into consideration. This means less processing.

heinob Over a year ago

But that should happen with '$all' also, shouldn't it!?

Evan Carroll Over a year ago

$all is probably two lookups on an index, $and is probably one lookup with a sequential scan on the result.

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