Unique array value in Mongo - mongodb

Question

I'm having a hard time to find a way to make a collection index work the way I need. That collection has an array that will contain two elements, and no other array can have these two elements (in any order):
db.collection.insert(users : [1,2] // should be valid
db.collection.insert(users : [2,3] // should be valid
db.collection.insert(users : [1,3] // should be valid
db.collection.insert(users : [3,2] // should be invalid, since there's another array with that same value.
But, if I use db.collection.createIndex({users:1}, {unique: true}), it won't allow me to have two arrays with a common element:
db.collection.insert(users : [1,2] // valid
db.collection.insert(users : [2,3] // invalid, since 2 is already on another document
One of the solutions I tried was to make the array one level deeper. Creating the very same index, but adding documents a little different would make it almost the way I need, but it would still allow two arrays to have the same value in the reverse orders:
db.chat.insert({ users : { people : [1,2] }}) // valid
db.chat.insert({ users : { people : [2,3] }}) // valid
db.chat.insert({ users : { people : [2,1] }}) // valid, but it should be invalid, since there's another document with [1,2] array value.
db.chat.insert({ users : { people : [1,2] }}) // invalid
Is there a way to achieve this on a index level?

Answer 1

The mongodb doesn't create indexes on the entire array. But...
We want one atomic operation insert or update, and guarantee uniqueness of the array's content? Then, we need to calculate one feature which is the same for all permutations of the array's items, and create an unique index for it.
One way would be to sort array items (solves permutation problem) and concatenate them (creates one feature). The example, in javascript:
function index(arr) {
return arr.sort().join();
}
users1 = [1, 2], usersIndex1 = index(users1); // "1,2"
users2 = [2, 1], usersIndex2 = index(users2); // "1,2"
// Create the index
db.collection.ensureIndex({usersIndex: 1}, {unique: true});
//
db.collection.insert({users: users1, usersIndex: usersIndex1}); // Ok
db.collection.insert({users: users2, usersIndex: usersIndex2}); // Error
If the arrays are long, you can apply a hash function on the strings, minimizing the size of the collection. Though, it comes with a price of possible collisions.

Answer 2

You have to write a custom validation in the pre save hook:
Coffee Version
Pre Save Hook
chatSchema.pre('save', (next) ->
data = #
#.constructor.findOne {}, (err, doc) ->
return next err if err?
return next "Duplicate" if customValidation(data, doc) == false
return next()
)
Custom Validation
customValidation = (oldDoc, newDoc)->
#whatever you need
e.g. return !lodash.equal(oldDoc, newDoc)
Js Version
var customValidation;
chatSchema.pre('save', function(next) {
var data;
data = this;
return this.constructor.findOne({}, function(err, doc) {
if (err != null) {
return next(err);
}
if (customValidation(data, doc) === false) {
return next("Duplicate");
}
return next();
});
});
customValidation = function(oldDoc, newDoc) {
return !lodash.equal(oldDoc, newDoc);
};

Answer 3

You should first find the records and If no record available then insert that
db.chat.findOne({users: {$all: [3,2]}})
.then(function(doc){
if(doc){
return res.json('already exists');
} else {
db.chat.insert({users: [3,2]})
}
})
.catch(next);