How to delete duplicates from 2nd level of array in Mongo DB - mongodb

Question

I have duplicates in 2nd level of array.
In 2nd level array elements all the columns contains same data except _id. In Some cases _id also is same.
Please suggest how to delete duplicates from 2nd level of array by keeping one element.
From the below example we can treat AssessmentName as unique filed in Bhra array.
Mongo Version : 3.2.8
Example:
{
"_id" : ObjectId("592415c434810eeb63afe029"),
"Encounter" : [
{
"_id" : ObjectId("5846c6361489b54e402d76f6"),
"Bhra" : [
{
"_id" : "15445853419048538e0ba2cd",
"AssessmentId" : 1,
"AssessmentName" : "Major Depressive Episode",
"AssessmentOrder" : 1,
"IsContinue" : true,
"IsNotAssessed" : false,
"TotalScore" : 9,
"DepressionSeverity" : "Mild depression",
"AssessmentResult" : "Negative",
"Notes" : ""
},
{
"_id" : "15445853419048538e0ba2aa",
"AssessmentId" : 1,
"AssessmentName" : "Major Depressive Episode",
"AssessmentOrder" : 1,
"IsContinue" : true,
"IsNotAssessed" : false,
"TotalScore" : 9,
"DepressionSeverity" : "Mild depression",
"AssessmentResult" : "Negative",
"Notes" : ""
},
{
"_id" : "15445853419048538e0ba2aa",
"AssessmentId" : 1,
"AssessmentName" : "Major Depressive Episode",
"AssessmentOrder" : 1,
"IsContinue" : true,
"IsNotAssessed" : false,
"TotalScore" : 9,
"DepressionSeverity" : "Mild depression",
"AssessmentResult" : "Negative",
"Notes" : ""
}
]
}
]
}
Output should be as follows.
{
"_id" : ObjectId("592415c434810eeb63afe029"),
"Encounter" : [
{
"_id" : ObjectId("5846c6361489b54e402d76f6"),
"Bhra" : [
{
"_id" : "15445853419048538e0ba2cd",// any _id is fine
"AssessmentId" : 1,
"AssessmentName" : "Major Depressive Episode",
"AssessmentOrder" : 1,
"IsContinue" : true,
"IsNotAssessed" : false,
"TotalScore" : 9,
"DepressionSeverity" : "Mild depression",
"AssessmentResult" : "Negative",
"Notes" : ""
}
]
}
]
}
Thanks,
Rao

Answer 1

You can use aggregation. First, you need to $unwind data 2 times in order to reach to level 2 array and then use $group to only include distinct item in array. Finally, $project into required json format.
db.your_collection.aggregate([
{"$unwind":"$Encounter"},
{"$unwind":"$Encounter.Bhra"},
{"$group":{
_id: {
_id:"$_id",
"Encounter_id":"$Encounter._id",
"Encounter_Bhra_AssessmentId":"$Encounter.Bhra.AssessmentId",
"Encounter_Bhra_AssessmentName":"$Encounter.Bhra.AssessmentName",
"Encounter_Bhra_AssessmentOrder":"$Encounter.Bhra.AssessmentOrder",
"Encounter_Bhra_IsContinue":"$Encounter.Bhra.IsContinue",
"Encounter_Bhra_IsNotAssessed":"$Encounter.Bhra.IsNotAssessed",
"Encounter_Bhra_TotalScore":"$Encounter.Bhra.TotalScore",
"Encounter_Bhra_DepressionSeverity":"$Encounter.Bhra.DepressionSeverity",
"Encounter_Bhra_AssessmentResult":"$Encounter.Bhra.AssessmentResult",
"Encounter_Bhra_Notes":"$Encounter.Bhra.Notes"
},
"Encounter_Bhra_id":{"$min":"$Encounter.Bhra._id"}
}
},
{"$project":{
_id:"$_id._id",
Encounter : {
_id:"$_id.Encounter_id",
Bhra : {
_id : "$Encounter_Bhra_id",
AssessmentId : "$_id.Encounter_Bhra_AssessmentId",
AssessmentName : "$_id.Encounter_Bhra_AssessmentName",
AssessmentOrder : "$_id.Encounter_Bhra_AssessmentOrder",
IsContinue : "$_id.Encounter_Bhra_IsContinue",
IsNotAssessed : "$_id.Encounter_Bhra_IsNotAssessed",
TotalScore : "$_id.Encounter_Bhra_TotalScore",
DepressionSeverity : "$_id.Encounter_Bhra_DepressionSeverity",
AssessmentResult : "$_id.Encounter_Bhra_AssessmentResult",
Notes : "$_id.Encounter_Bhra_Notes"
}
}
}
},
{$out: "your_new_coll_name"}
])
Output:
{
"_id" : ObjectId("592415c434810eeb63afe029"),
"Encounter" : {
"_id" : ObjectId("5846c6361489b54e402d76f6"),
"Bhra" : {
"_id" : "15445853419048538e0ba2aa",
"AssessmentId" : 1,
"AssessmentName" : "Major Depressive Episode",
"AssessmentOrder" : 1,
"IsContinue" : true,
"IsNotAssessed" : false,
"TotalScore" : 9,
"DepressionSeverity" : "Mild depression",
"AssessmentResult" : "Negative",
"Notes" : ""
}
}
}