MongoDB find if property in array of objects equals to another field

Question

This is the document model:

{
  foo: {
    bar: 1
  },
  some_array: [
    {
      xyz: 1
    },
    {
      xyz: 2
    }
  ]
},

{
  foo: {
    bar: 2
  },
  some_array: [
    {
      xyz: 123
    },
    {
      xyz: 321
    }
  ]
}

I want to find the documents that 'foo.bar' equals to 'some_array.xyz'.

As the example, only the first document should be fetched.

How to do it?

Montgomery Watts · Accepted Answer · 2022-01-14 21:48:46Z

2

You can use the $expr operator to compare fields within the same document.

db.collection.find({
  $expr: {
    $in: [
      "$foo.bar",
      "$some_array.xyz"
    ]
  }
})

Working MongoPlayground

answered Jan 14, 2022 at 21:48

Montgomery Watts

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4 Comments

Raz Buchnik Over a year ago

Will it work on aggregation as well?

Montgomery Watts Over a year ago

Yes. MongoPlayground

Raz Buchnik Over a year ago

When I try the opposite - $nin it doesn't work I got an error: uery failed: (InvalidPipelineOperator) Unrecognized expression '$nin'

Montgomery Watts Over a year ago

$expr uses aggregation expressions, which $nin is not. You can achieve the same result by wrapping $in with $not.

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