MongoDB: Get count of inner array object with nested array element match - spring

Question

I have mongo collection with survey answers submitted by each user. I would like to get the count of users selected as an option. Only one user has selected the option O12. The output should be 1.
{
"_id" : ObjectId("5ea179eb39ff117948f19266"),
"_class" : "model.survey.Answer",
"survey_id" : "5ea178c239ff117948f19265",
"survey_user" : [
{
"user_id" : 1072,
"user_option" : [
{
"question_id" : "Q1",
"option_id" : "O11"
},
{
"question_id" : "Q2",
"option_id" : "O21"
},
{
"question_id" : "Q3",
"option_id" : "O31"
},
{
"question_id" : "Q4",
"option_id" : "O41"
}
]
},
{
"user_id" : 1073,
"user_option" : [
{
"question_id" : "Q1",
"option_id" : "O12"
},
{
"question_id" : "Q2",
"option_id" : "O21"
},
{
"question_id" : "Q3",
"option_id" : "O31"
},
{
"question_id" : "Q4",
"option_id" : "O41"
}
]
}
]
}

Answer 1

You can do that using MongoDB's aggregation-pipeline :
Different ways to do it, One way is to use $unwind:
Type 1 - Query 1 :
db.collection.aggregate([
/** Optional but will be good on huge collections to lessen data for further stages */
{
$match: { "survey_user.user_option.option_id": "O12" }
},
{
$unwind: "$survey_user"
},
/** When you unwind a each object/element in array gets it's own document after `unwind` stage */
{
$match: { "survey_user.user_option.option_id": "O12" }
},
/** After match you'll only have objects which met the criteria in `survey_user` array */
/** group on `_id` & push entire original doc to data field */
{
$group: { _id: "$_id", survey_user: { $push: "$survey_user" }, data: { $first: "$$ROOT" } }
},
/** Add `survey_user` array to `data.survey_user` & it's size to `data.optedCount` field */
{
$addFields: { "data.survey_user": "$survey_user", "data.optedCount": { $size: "$survey_user" } }
},
/** Make `data` as new root to doc */
{
$replaceRoot: { newRoot: "$data" }
}
])
Test : mongoplayground
Just in case if you just need count but not needed the entire doc to be returned there will be a minor change in above query :
Type 1 - Query 2 :
db.collection.aggregate([
{
$match: { "survey_user.user_option.option_id": "O12" }
},
{
$unwind: "$survey_user"
},
{
$match: { "survey_user.user_option.option_id": "O12" }
},
/** Just group on `_id` & count no.of docs, maintain `survey_id` */
{
$group: { _id: "$_id", optedCount: { $sum: 1 }, survey_id: { $first: "$survey_id" } }
}
])
Test : mongoplayground
Using array iterator $reduce, which might be helpful if your collections data is so huge, as unwind will explode your docs.
Type 2 - Query :
db.collection.aggregate([
{
$match: {
"survey_user.user_option.option_id": "O12",
},
},
/** Instead of `$addFields`, you can use `$project` to project fewer needed fields (which can be help improve query with performance benefits ) */
{
$addFields: {
optedCount: {
$reduce: {
input: "$survey_user",
initialValue: 0,
in: {
$cond: [
{ $in: ["O12", "$$this.user_option.option_id"] },
{ $add: ["$$value", 1] },
"$$value",
]
}
}
}
}
}
]);
Test : mongoplayground