mongodb distinct the key of a map in an array which is a property of a document

I use mongodb,one document format is like this:


sid:2,
attr:[

key:"name",
value:"bike"
,
key:"weight",
value:"100"

]



there is a property "attr" which is like a map with uncertain number of keys.
the whole collection look like this:

[
sid:1,
attr:[

key:"name",
value:"bike"

]
,
sid:2,
attr:[

key:"name",
value:"bike"
,
key:"weight",
value:"100"

]
,
sid:3,
attr:[

key:"color",
value:"red"
,
key:"weight",
value:"100"

]
]


Now, what I want to know is how many distinct keys there are,and what are them.
In this example,the distinct keys I expected should be:

["name","weight","color"]


but what about more document inserted into the collection?The data in this collection is not fixed

db.colname.distinct("attr.key")

1 Answer
1

You need to first $unwind the attr array then you can use $group aggregation to distinct the fields.

$unwind

attr

$group

db.collection.aggregate([
"$unwind": "$attr" ,
"$group": "_id": "$attr.key"
])


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