Using $addToSet to update an array field using another array field - mongodb

Question

I should start with: I'm knew to MongoDB, and document-style databases in general.
I have a collection that looks something like this:
{
"_id" : ObjectId("554a5e72b16f31ff0894310e"),
"title" : "ABC",
"admins" : [
"personA",
"personB",
],
"email_address" : "ABC#mysite.com"
}
{
"_id" : ObjectId("554a5e72b16f31ff0894310f"),
"title" : "Junk Site",
"admins" : [
"personA",
"personB"
],
"email_address" : "garbage#mysite.com"
}
{
"_id" : ObjectId("554a5e72b16f31ff08943110"),
"title" : "Company Three Site",
"admins" : [
"personC"
"personD",
],
"email_address" : "company2plus1#mysite.com"
}
What I need to do, is append the admins list from Company One, to Company Three such that Company Three now has four admins (A, B, C, D).
I tried the following, because it seemed pretty straight forward to me - get the data from the origin and append to destination directly:
db.runCommand({
findAndModify : 'sites',
query : {'title' : 'Company Three Site'},
update : { '$addToSet' :
{'admins' :
db.projects.find({'title' : 'ABC'}, {'_id' : 0, 'admins' : 1}
}
}
})
However, this does not work correctly.
I am still trying to figure out ways I could do this directly, but questions...
1) Is this even possible by using single command, or do I need to split this up?
2) Does my train of logical thought make sense, or should I be doing this some other/easier way that is more conventional for MongoDB style databases?

Answer 1

db.projects.find actually returns a cursor, which you definitely don't want to add to your set. Since you know ahead of time that you will be only finding one value, you can get the properties out of the cursor specifically by using .next().admin -- but remember that this will only work with the first value returned from .find. Otherwise, I think you will have to use a loop.
$addToSet will also add the array as a whole, so instead you have to append multiple values using $each
All together:
db.runCommand({
findAndModify: 'sites',
query: {'title': 'Company Three Site'},
update: {
$addToSet: {
"admins": {
$each: db.projects.find(
{"title": "ABC"},
{"_id": 0, "admins": 1}
).next().admins
}
}
}
})

Answer 2

This is not possible with an atomic update. However, a workaround is to query the source collection using the find() method and use the cursor's forEach() method to iterate over the results, get the array and update the destination collection using the $addToSet operator and the $each modifier.
Let's demonstrate this with the above sample documents inserted to a test collection:
db.test.insert([
{
"title" : "ABC",
"admins" : [
"personA",
"personB"
],
"email_address" : "ABC#mysite.com"
},
{
"title" : "Junk Site",
"admins" : [
"personA",
"personB"
],
"email_address" : "garbage#mysite.com"
},
{
"title" : "Company Three Site",
"admins" : [
"personC",
"personD"
],
"email_address" : "company2plus1#mysite.com"
}
])
The following operation will add the admins array elements from company "ABC" to the company "Company Three Site" admin array:
db.test.find({"title" : "ABC"}).forEach(function (doc){
var admins = doc.admins;
db.test.update(
{"title" : "Company Three Site"},
{
"$addToSet": {
"admins": { "$each": admins }
}
},
{ "multi": true }
);
});
Querying the collection for the document with company "Company Three Site" db.collection.find({"title" : "Company Three Site"});
will yield:
/* 0 */
{
"_id" : ObjectId("554a7dc35c5e0118072dd885"),
"title" : "Company Three Site",
"admins" : [
"personC",
"personD",
"personA",
"personB"
],
"email_address" : "company2plus1#mysite.com"
}