A unique index on an array field enforces that the same value cannot appear in the arrays of more than one document in the collection, but doesn’t prevent the same value from appearing more than once in a single document’s array. So you need to ensure uniqueness as you add elements to the array instead.
Use the $addToSet
operator to add a value to an array only if the value is not already present.
Group.updateOne({name: 'admin'}, {$addToSet: {users: userOid}}, ...
However, if the users
array contains objects with multiple properties and you want to ensure uniqueness over just one of them (uid
in this case), then you need to take another approach:
var user = { uid: userOid, ... };
Group.updateOne(
{name: 'admin', 'users.uid': {$ne: user.uid}},
{$push: {users: user}},
function(err, numAffected) { ... });
What that does is qualify the $push
update to only occur if user.uid
doesn’t already exist in the uid
field of any of the elements of users
. So it mimics $addToSet
behavior, but for just uid
.