Move a Mongo sub collection to your own collection

Question

Can someone point me in the right direction, I have Collection (Forms), each form has a built-in array of documents (Answers). The answers for each form became widespread, and in retrospect they were a bad idea (mongo documents, including embedded ones, have a maximum size).

Is there a way that I can quickly and easily move all of these inline answers to my own collection? is there such a thing as an old SQL select? I looked around in the rails console, but it is not available for many embedded documents, so I assume it will be difficult to search and insert a query in the mongo console? (just guessing)

My model is fixed, but this migration (and mongo docs) throws me away.

Tia Dougle

Answer 1

So, here is the beginning ... It's in the mongo shell

db.questions.insert({name:"jwo", responses:[{question:"your name?", answer:"yomamma"}, {question:"your name?", answer:"pappa"}]});

This created the json structure of the document, for example:

> db.questions.findOne();
{
    "_id" : ObjectId("4d877e89b75dc42c4709278d"),
    "name" : "jwo",
    "responses" : [
        {
            "question" : "your name?",
            "answer" : "yomamma"
        },
        {
            "question" : "your name?",
            "answer" : "pappa"
        }
    ]
}

Now navigate through the answers and set them to question_id with questions _id, and then paste it into a new collection of answers

> for(i=0; i<question.responses.length; ++i){
... question.responses[i].question_id = question._id;   
... db.responses.insert(question.responses[i]);                                                                      
... }

> db.responses.findOne();
{
    "_id" : ObjectId("4d878059b75dc42c4709278e"),
    "question" : "your name?",
    "answer" : "yomamma",
    "question_id" : ObjectId("4d877e89b75dc42c4709278d")
}

You will want to modify db.questions.findOne to find all of them and execute the loop. If this takes some time, you may need to switch to the map reduction function.

Answer 2

Here is the code we ended up in based on Jesse Wolgamott's answer.

var count = 0;
db.transactions.find().sort({_id: 1}).forEach(function(t){
  if(count % 10000 == 0)
    print(""+t._id+" "+count);

  count += 1;

  for(i=0; i<t.inputs.length; ++i){
    t.inputs[i].transaction_id = t._id;
    db.input2s.insert(t.inputs[i]);
  }
});