Is it encouraged to query a MongoDB database in a loop?

Question

In an SQL database, if I wanted to access some sort of nested data, such as a list of tags or categories for each item in a table, I'd have to use some obscure form of joining in order to send the SQL query once and then only loop through the result cursor.

My question is, in a NoSQL database such as MongoDB, is it OK to query the database repeatedly such that I can do the previous task as follows:

cursor = query for all items
for each item in cursor do
    tags = query for item's tags

I know that I can store the tags in an array in the item's document, but I'm assuming that it is somehow not possible to store everything inside the same document. If that is the case, would it be expensive to requery the database repeatedly or is it designed to be used that way?

Answer 1

No, neither in Mongo, nor in any other database should you query a database in a loop. And one good reason for this is performance: in most web apps, database is a bottleneck and devs trying to make as small amount of db calls as possible, whereas here you are trying to make as many as possible.

I mongo you can do what you want in many ways. Some of them are:

• putting your tags inside the document {itemName : 'item', tags : [1, 2, 3]}
• knowing the list of elements, you do not need a loop to find information about them. You can fetch all results in one query with $in : db.tags.find({ field: { $in: [<value1>, <value2>, ... <valueN> ] }})

Answer 2

You should always try to fulfill a request with as few queries as possible. Keep in mind that each query, even when the database can answer it entirely from cache, requires a network roundtrip between application server, database and back.

Even when you assume that both servers are in the same datacenter and only have a latency of microseconds, these latency times will add up when you query for a large number of documents.

Relational databases solve this issue with the JOIN command. But unfortunately MongoDB has no support for joins. For that reason you should try to build your documents in a way that the most common queries can be answered by a single document. That means that you should denormalize your data. When you have a 1:n relation, you should consider to embed the referencing documents as an array in the main document. Having redundancies in your database is usually not as unacceptable in MongoDB as it is in relational databases.

When you still have good reasons to keep the child-documents as separate documents, you should use a query with the $in operator to query them all at once, as Salvador Dali suggested in his answer.