Does not contain _id or shard key for pattern {_id:hashed}

Question

db.books.update({bookId:"123461"},{$set:{"bookPrice":"6.23"}})

I get the error:

   update { q: { bookId: "123461" }, u: { $set: { bookPrice: "6.23" } }, multi: false, upsert: false } does not contain _id or shard key for pattern { _id: "hashed" }

But when I use below it works.

 db.books.update({_id:ObjectId("54b88167498ec382221a82c2")},{$set:     {"bookPrice":"6.23"}})

Why doesn't it work with the bookId

Answer 1

The reason is because you are on a sharded cluster and you are identifying the document you wish to update with an index that is neither unique nor is it the shard key however you have specified that you only want to update 1 document. (multi: false)

Consider that when you make a query that does not include the shard key, mongodb has to scatter the query to ALL of the shards because there is no way for mongos to figure out which shard the document might be on.

So if mongos were to broadcast your query to all of the shards, two or more of them may find a document that matches your query and they would both update the document they found. This would violate your {multi: false}.

Now perhaps you know that bookId is a unique identifier, but your mongodb cluster does not. Is there any chance that you could replace bookId with the _id? That is, could you change the document so that instead of having a bookId field, it has _id: "123461"? Or if you know bookId is unique you could just set multi: true. Though it will not be an efficient operation since the command will have to be sent to all of the shards even though the document is only in one of them.