Querying internal array size in MongoDB

mongodb mongodb-query nosql
Zaur Nasibov picture Zaur Nasibov · Jul 17, 2011 · Viewed 34.2k times · Source

Consider a MongoDB document in users collection: 

{ username : 'Alex', tags: ['C#', 'Java', 'C++'] }

Is there any way, to get the length of the tags array from the server side (without passing the tags to the client) ?

Thank you!

Answer

xmm.dev picture xmm.dev · Dec 15, 2012

if username Alex is unique, you can use next code:

db.test.insert({username:"Alex", tags: ['C#', 'Java', 'C++'] });
db.test.aggregate(
  {$match: {username : "Alex"}}, 
  {$unwind: "$tags"},
  {$project: {count:{$add:1}}},
  {$group: {_id: null, number: {$sum: "$count" }}}
);
{ "result" : [ { "_id" : null, "number" : 3 } ], "ok" : 1 }

Related questions

MongoDB nested array query

I've asked this as a comment on another question, and also posted a question on mongodb-user. No responses so far, so I'm resorting to asking a separate question. The documentation states: If the field holds an array, then the $in …

Read More
How to stop insertion of Duplicate documents in a mongodb collection

Let us have a MongoDB collection which has three docs.. db.collection.find() { _id:'...', user: 'A', title: 'Physics', Bank: 'Bank_A' } { _id:'...', user: 'A', title: 'Chemistry', Bank: 'Bank_B' } { _id:'...', user: 'B', title: 'Chemistry', …

Read More
Update field in exact element array in MongoDB

I have a document structured like this: { _id:"43434", heroes : [ { nickname : "test", items : ["", "", ""] }, { nickname : "test2", items : ["", "", ""] }, ] } Can I $set the second element of the items array of the embedded object in array heros with nickname "test" ? Result: { _id:"43434", heroes : [ { nickname : "test", …

Read More