具有一个字段未知的MongoDB复合密钥查询

Question

I have a database that has a unique combination of two fields (x and i) for every entry. So I have set the _id field to be {_id: {a: x, b: i}}. Now I want to retrieve all values that have a certain value x but that have any value for i.

Example:

  {_id: {a: 1, b: 5}},
  {_id: {a: 1, b: 3}},
  {_id: {a: 2, b: 5}}
  {_id: {a: 3, b: 3}}

Now I want to do something like: db.find({_id: {a: 1, b: { $exists: true}}) or even easier: db.find({_id: {a: 1}) that should return:

  {_id: {a: 1, b: 5}},
  {_id: {a: 1, b: 3}}

Is there any way I could achieve this? Or in other words can you query in any way on this composite primary key? Currently I added the fields to the object itself but this is not really an optimal solution as my data set gets really large.

Edit:

db.someCollection.find({"_id.a": 1, "_id.b": { $exists: true}})

Seems to be a solution, this is however just as slow as adding a as a field (not in the key) to the object. Is there a faster method?

Answer 1

你有尝试过吗？

db.someCollection.find({"_id.a": 1, "_id.b": { $exists: true}})