Retrieve only the needed element from an object array in MongoDB collection

Question

I am working with MongoDB for the first time. I want to retrieve a particular field from an object array in MongoDB collection. I need help how to do this?

In my database there is a structure called skills which is in the form of array. I want to retrieve both the skillName and experienceInMonths individually.

{
"skills" : [
        {
            "skillName" : "sql",
            "experienceInMonths" : 2
        },
        {
            "skillName" : "java",
            "experienceInMonths" : 3
        }
    ]
}

I tried to retrieve the particular field using $elemMatch.

But, I am not getting the expected result.

> db.userforms.find({"emailId": "[email protected]"},{"_id":0, "skills":1})
{ "skills" : [ { "skillName" : "sql", "experienceInMonths" : 2 }, { "skillName" : "java", "experienceInMonths" : 3 } ] }

> db.userforms.find({"emailId":"[email protected]"},{"_id":0, "skills":{$elemMatch: {"skillName":"sql"}}})
{ "skills" : [ { "skillName" : "sql", "experienceInMonths" : 2 } ] }

My actual result is: { "skills" : [ { "skillName" : "sql", "experienceInMonths" : 2 } ] }

But the required result is: {"skillName" : ["sql", "java"]}

try this { $project: { skillName: "$skills.skillName" } }

Ashh
– Ashh

2019-01-09 17:22:36 +00:00
Commented Jan 9, 2019 at 17:22 — Ashh
– Ashh, Commented Jan 9, 2019 at 17:22

Ionut Achim · Accepted Answer · 2019-01-09 13:41:52Z

2

Use aggregation:

db.userforms.aggregate([
  { $match: { emailId: "[email protected]" } },
  { $project: { skillName: { $concatArrays: "$skills.skillName" } } }
])

will return:

{ "_id" : 0, "skillName" : [ "sql", "java" ] }

edited Jan 9, 2019 at 13:41

answered Jan 9, 2019 at 12:28

Ionut Achim

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3 Comments

Pynchia Over a year ago

Dolly wants "skillname", not "skills" ;)

Ionut Achim Over a year ago

'skillName' will be. I misspelled the return. (edited)

Ionut Achim Over a year ago

Are you querying userforms? I wrote db.users the first time but then I edited. It should work, I've checked.

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