mongodb index an array's key (not the values)

Question

In MongoDB, I have the following document

{
  "_id": { "$oid" : "4FFD813FE4B0931BDAAB4F01" },
  "concepts": {
    "blabla": 20,
    "blibli": 100,
    "blublu": 250,
    ... (many more here)
  }
}

And I would like to index it to be able to query for the "key" of the "concept" array (I know it's not really a mongoDB array...):

db.things.find({concepts:blabla});

Is it possible with the above schema? Or shall I refactor my documents to something like

{
  "_id": { "$oid" : "4FFD813FE4B0931BDAAB4F01" },
  "concepts": ["blabla","blibli","blublu", ... (many more here)]
  }
}

Remon van Vliet · Accepted Answer · 2012-08-16 13:20:25Z

2

I'll answer your actual question. No you cannot index on the field names given your current schema. $exists uses an index but that is an existence check only.

There are a lot of problems with a schema like the one you're using and I would suggest a refactor to :

{
  "_id": { "$oid" : "4FFD813FE4B0931BDAAB4F01" },
  "concepts": [
    {name:"blabla", value: 20},
    {name:"blibli", value: 100},
    {name:"blublu", value: 250},
    ... (many more here)
  ]
}

then index {'concepts.name:1'} and you can actually query on the concept names rather than just check for the existence.

TL;DR : No you can't.

answered Aug 16, 2012 at 13:20

Remon van Vliet

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Aurélien B · Accepted Answer · 2012-08-16 15:59:37Z

1

You can query field presence with specific query:

db.your_collection.find({"concept.yourfield": { $exists: true }})

(notice the $exists)

It will return all your document where yourfield is a field of concept subdocument

edit: this solution is only about query. Indexes contains values not field.

edited Aug 16, 2012 at 15:59

answered Aug 16, 2012 at 10:17

Aurélien B

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4 Comments

Renaud Over a year ago

Thanks Aurelien. How do I make MongoDB index only the keys (not the values)?

Remon van Vliet Over a year ago

You can't. Keys are never part of index data and you should never have a schema where keys are data.

Aurélien B Over a year ago

Oups I didnt read enough about the index. So yes, you can still use my method to find only specific document but you cannot index, as Remon said. Except if you create an index on each field of concept, which is probably not a nice idea.

Renaud Over a year ago

Thanks guys. Remon, you are right -- we should not use a schema where keys are data.

Erhan A · Accepted Answer · 2012-08-16 10:07:05Z

0

MongoDB indexes each value of the array so you can query for individual items.As you can find here.

But in nested arrays you need to tell to index mongodb to index your sub-fields.

  db.col1.ensureIndex({'concepts.blabla':1})
  db.col1.ensureIndex({'concepts.blublu':1})
    db.col1.find({'concepts.blabla': 20}).explain()
    {
        "cursor" : "BtreeCursor concepts.blabla_1",
        "nscanned" : 1,
        "nscannedObjects" : 1,
        "n" : 1,
        "millis" : 0,
        "nYields" : 0,
        "nChunkSkips" : 0,
        "isMultiKey" : false,
        "indexOnly" : false,
        "indexBounds" : {
            "concepts.blabla" : [
                [
                    20,
                    20
                ]
            ]
        }
    }

After creating the index , the cursor type changes itself from BasicCursor to BtreeCursor.

if you create your document as you stated at the end of your question

{
  "_id": { "$oid" : "4FFD813FE4B0931BDAAB4F01" },
  "concepts": ["blabla","blibli","blublu", ... (many more here)]
  }
}

just the indexing will be enough as below:

db.col1.ensureIndex({'concepts':1})

answered Aug 16, 2012 at 10:07

Erhan A

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2 Comments

Renaud Over a year ago

Maybe I was not clear, but I am only interested in the presence of keys (here: blabla, blibli), not in the values (here: 20, 100).

Erhan A Over a year ago

the indexed keys can presence only if the the key has a value ( even null).Else there is no meaning of indexes if you will not use it in query for upsert or find.You will just increase memory consumption.

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