All Mongo documents with duplicated objects inside array - mongodb

Question

Find documents with duplicated objects inside an array.
Some answers works just with array made of "basic type elements" (i.e. array of strings). Here I want to filter on certain objects fields
In example:
{
"name": "1",
"arr": [{ "type": "fruit", "name":"pear"},{ "type": "fruit","name":"banana"}]
},
{
"name":"2",
"arr": [{"type":"fish"}]
}
Given the above two documents, I want to retrieve just document 1, because it has 2 elements in the array that have the same type. (Of course I want all documents with such property, not just one)

Answer 1

The following query can get us the expected output:
db.collection.find({
$expr:{
$ne:[
{
$size:"$arr"
},
{
$size:{
$setUnion:["$arr.type"]
}
}
]
}
}).pretty()
Data set:
{
"_id" : ObjectId("5d7b8546d76ccfa3cb0f133c"),
"name" : "1",
"arr" : [
{
"type" : "fruit",
"name" : "pear"
},
{
"type" : "fruit",
"name" : "banana"
}
]
}
{
"_id" : ObjectId("5d7b8546d76ccfa3cb0f133d"),
"name" : "2",
"arr" : [
{
"type" : "fish"
}
]
}
{
"_id" : ObjectId("5d7b8546d76ccfa3cb0f133e"),
"name" : "3",
"arr" : [
{
"type" : "product",
"name" : "watch"
},
{
"type" : "product",
"name" : "Pen"
}
]
}
Output:
{
"_id" : ObjectId("5d7b8546d76ccfa3cb0f133c"),
"name" : "1",
"arr" : [
{
"type" : "fruit",
"name" : "pear"
},
{
"type" : "fruit",
"name" : "banana"
}
]
}
{
"_id" : ObjectId("5d7b8546d76ccfa3cb0f133e"),
"name" : "3",
"arr" : [
{
"type" : "product",
"name" : "watch"
},
{
"type" : "product",
"name" : "Pen"
}
]
}
Query analysis: We are filtering documents in which the size of arr is not equal to the count of unique type present in the arr