mongdb ensure uniqueness on two fields both ways - mongodb

Question

Say I have the fields a and b. I want to have a compound uniqueness where if a: 1, b: 2, I would not be able to do a: 2, b: 1.
The reason I want this is because I'm making a "friends list" kind of collection, where if a is connected to b, then it's automatically the reverse as well.
is this possible on a schema level or do I need to do queries to check.

Answer 1

If you don't need to differentiate between requester and requestee, you could sort the values before saving or querying so that your two fields a and b have a predictable order for any pair of friend IDs (and you can take advantage of the unique index constraint).
For example, using the mongo shell:
Create a helper function to return friend pairs in predictable order:
function friendpair (friend1, friend2) {
if ( friend1 < friend2) {
return ({a: friend1, b: friend2})
} else {
return ({a: friend2, b: friend1})
}
}
Add a compound unique index:
> db.friends.createIndex({a:1, b:1}, {unique: true});
{
"createdCollectionAutomatically" : true,
"numIndexesBefore" : 1,
"numIndexesAfter" : 2,
"ok" : 1
}
Insert unique pairs (should work)
> db.friends.insert(friendpair(1,2))
WriteResult({ "nInserted" : 1 })
> db.friends.insert(friendpair(1,3))
WriteResult({ "nInserted" : 1 })
Insert non-unique pair (should return duplicate key error):
> db.friends.insert(friendpair(2,1))
WriteResult({
"nInserted" : 0,
"writeError" : {
"code" : 11000,
"errmsg" : "E11000 duplicate key error collection: test.friends index: a_1_b_1 dup key: { : 1.0, : 2.0 }"
}
})
Search should work in either order:
db.friends.find(friendpair(3,1)).pretty()
{ "_id" : ObjectId("5bc80ed11466009f3b56fa52"), "a" : 1, "b" : 3 }
db.friends.find(friendpair(1,3)).pretty()
{ "_id" : ObjectId("5bc80ed11466009f3b56fa52"), "a" : 1, "b" : 3 }
Instead of handling duplicate key errors or insert versus update, you could also use findAndModify with an upsert since this is expected to be a unique pair:
> var pair = friendpair(2,1)
> db.friends.findAndModify({
query: pair,
update: {
$set: {
a : pair.a,
b : pair.b
},
$setOnInsert: { status: 'pending' },
},
upsert: true
})
{
"_id" : ObjectId("5bc81722ce51da0e4118c92f"),
"a" : 1,
"b" : 2,
"status" : "pending"
}

Answer 2

Doesn't seem like you can do a unique on the entire array's values so I'm doing a kind of work around. I'm using the $jsonSchema as follows:
{
$jsonSchema:
{
bsonType:
"object",
required:
[
"status",
"users"
],
properties:
{
status:
{
enum:
[
"pending",
"accepted"
],
bsonType:
"string"
},
users:
{
bsonType:
"array",
description:
"references two user_id",
items:
{
bsonType:
"objectId"
},
maxItems:
2,
minItems:
2,
},
}
}
}
then I will use $all to find the connected users, e.g.
db.collection.find( { users: { $all: [ ObjectId1, ObjectId2 ] } } )