您如何每次递增一个字段，而仅当它大于某个具有multi true的数时才递增另一个字段？

Question

The following code decrements class_number just fine, but not number_of_classes because of the $gt operator.

posts.update({ 'title':doc.title, 'author':doc.author, 'class_number':{ '$gt': doc.class_number } }, 
             { '$inc': { 'number_of_classes':-1 , 'class_number':-1} }, 
             { multi: true }, function(err, dox){....

I need to decrement number_of_classes once every time, but only each class_number above a certain value. Is there a way to do this in one statment?

Answer 1

No there is no way to "conditionally" update one field and not another, just as there is no way to "reference" an existing value of a field in a document during an update statement. Much as your current statement provides, you can only perform the "conditions" on the query that you pass to the update.

So if you are trying to hit a "floor" of 0 then your "safe" method is in two operations:

db.collection.update(
     { "title":doc.title, "author":doc.author },
     { "$inc": "number_of_classes": -1 }
)

db.collection.update(
     { 
         "title":doc.title, 
         "author":doc.author,
         "$and": [
             { "class_number": { "$gt": 0 } },
             { "class_number": { "$gt": doc.class_number } }
         ]
     },
     { "$inc": "class_number": -1 }
)

Or however that logic works for you in actual updates.

In future versions ( as of writing ) you can "batch" these updates into a single request, but they actually are still two operations, even though sent to the server only once.

I do somehow suspect that due to the way you are writing this statement that you are actually trying to solve some kind of "allocation" problem. And that would be a different question to the way you are asking this here.