how to query two collections in once with mongodb - php

Question

I have two collections like this:
var Users = [
{
'ID' : 1,
'NAME' : 'Seth'
},
{
'ID' : 2,
'NAME' : 'John'
}
];
var Posts = [
{
'ID' : 1,
'TEXT' : 'blalalalalala...',
'USER' : 1
},
{
'ID' : 3,
'TEXT' : 'blalalalalala....',
'USER' : 2
}
];
How can I get the posts created by Seth without his ID? (Query By NAME : Seth)

Answer 1

If this is your schema, then there is no way of getting the posts for a given user by name in one query. In your application, you would have to perform two queries. The first to get the ID given a NAME and the second to get the posts given that USER. This is referred to in the MongoDB documentation as "manual referencing" and it would work something like this:
var id = db.Users.findOne({ "NAME" : "Seth" });
var posts = db.Posts.find({ "USER" : id });
If you indeed want to do this with one query, then you will have to look into embedding posts in an array inside the user documents, using DBRefs (which you need to be careful with), or in the case of some ODMs like Mongoose, using "population".
UPDATE:
Given the new information presented by the OP, in the case where you want to find all posts for users that have not been deleted, and if you don't want to iterate through a large array every time, I would suggest adding a USER_DELETED field that can be set to true when a user is deleted.
So in the event that a user with { "ID" : id } is deleted, you would run a query as follows:
db.Posts.update({ "USER" : id }, { "$set" : { "USER_DELETED" : true } });
That would then mark all of the posts for that user as belonging to a user that has been deleted. So when you want to retrieve the posts for all users that are still around, you would run:
db.Posts.find({ "USER_DELETED" : { "$ne" : true } });
Or you could do something along those lines. If you know that all the documents have a USER_DELETED field set to false if the user is still around, then you can just query for those that have the flag set to false.
Of course you will still have to transform the old data to follow this pattern, but that can be achieved with a one time operation that consists of:
Finding all of the users that have been deleted.
Finding the posts for each of those users.
Adding the USER_DELETED flag for each of those posts.
That should be a better way of approaching the problem you are facing.