MongoDB : regex for a specific element in an array

Question

Let say we have a collection structured like this:

{
  _id:1,
  tag:["speaker","tom"]
},
{
  _id:2,
  tag:["subject","tomatoes"]
}
{
  _id:3,
  tag:["subject","space"]
}

I can query for specific elements, such as :

db.tags.find({tag:["speaker","tom"]})

Now is it possible to perform a query with a regex matching the second element of that array ? something like :

db.tags.find({tag:["speaker",/tom.*/]}) ?

A regex on the field tag (ie : db.tags.find({tag:/sp.*/})) would match the entire array, and I'm looking for a way to make it search only on the second one... before considering to have to restructure that collection.

Update

A workaround is to use the $all operator :

db.tags.find({tag:{$all : ["speaker",/tom.*/]} })

which is equivalent to :

db.tags.find({ $and: [ { tag: "speaker }, { tags: /tom.*/ } ] }

However, that's not the correct way to do, since both element are scanned for both the string and the regex.

ThrowsException · Accepted Answer · 2016-08-30 02:18:58Z

1

Allow me to redeem myself. I found you can do it this way using the aggregation framework

db.tags.aggregate([
    {$match: {"tag.1" : {$exists: 1}}},
    {$project: {one: {$slice: ["$tag", 1]}, two: {$slice: ["$tag", 1,2]}}},
    {$match: {one: "speaker", two: /tom.*/}}
])

first part make sure the array has atleast 2 elements. the project the first element onto one and the second element onto two using slice then match against the single element in each array now.

You can then either get the ID and go back to the original collection and run another query or add some more projections to this and extract the necessary data from the aggregation framework.

edited Aug 30, 2016 at 2:18

answered Aug 30, 2016 at 0:27

ThrowsException

2,64622 silver badges38 bronze badges

Sign up to request clarification or add additional context in comments.

1 Comment

ThrowsException Over a year ago

Yea Sorry. I assumed since this query works db.tags.find({"tag.1": {$exists: 1}}) that you'd be able to do something similar with a find but that does not seem to be the case.

Collectives™ on Stack Overflow

MongoDB : regex for a specific element in an array

1 Answer 1

1 Comment

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

1 Comment

Your Answer

Sign up or log in

Post as a guest

Related