MongoDB aggregation query null and value

Question

I store a couple of content objects in a mongodb collection:

{ ..., category: "example1", collection: 'collectionX' },
{ ..., category: "example2", collection: null },
{ ..., category: "example1", collection: null }

I want to query the third option by the following query:

[
  { '$match': { 'category': 'example1' } },
  { '$match': { 'collection': null } }
]

Unfortunately, the result is empty. The query for both options separately returns the expected result. What is wrong?

Answer 1

等待大神答复