MongoDB get all documents where array values match

Question

I have the following structure of document:

  {
    "input": {
      "fields": [
        {
          "name": "last_name_hebrew",
          "text": "test1",
        },
      ],
    },
    "output": {
      "fields": [
        {
          "name": "last_name_hebrew",
          "text": "test1"
        },
      ],
    },
  },

I want to get all documents, where fields has object that has name of value last_name_hebrew as with text value of the output.fields.

For example in the given structure it would return this documents because input.fields.name is last_name_hebrew and text is equal to the text in output.

Note I cannot guarantee that fields array in either input or output will have name: last_name_hebrew in the array.

How can I do so?

This is my try to first force the arrays to have document with name of last_name_hebrew:

db.collection.find({
  "input.fields": {
    $elemMatch: {
      "name": "last_name_hebrew"
    }
  },
  "output.fields": {
    $elemMatch: {
      "name": "last_name_hebrew"
    }
  },
  
})

But now I need to compare the text values.

Answer 1

• Your first 2 condition with $elemMatch is correct
• add expression match, first find the matching element that having last_name_hebrew name from input using $filter and get first element from that filtered result using $arrayElemAt, same process for output field and then match both object using $eq

db.collection.find({
  "input.fields": { $elemMatch: { "name": "last_name_hebrew" } },
  "output.fields": { $elemMatch: { "name": "last_name_hebrew" } },
  $expr: {
    $eq: [
      {
        $arrayElemAt: [
          {
            $filter: {
              input: "$input.fields",
              cond: { $eq: ["$$this.name", "last_name_hebrew"] }
            }
          },
          0
        ]
      },
      {
        $arrayElemAt: [
          {
            $filter: {
              input: "$output.fields",
              cond: { $eq: ["$$this.name", "last_name_hebrew"] }
            }
          },
          0
        ]
      }
    ]
  }
});

Playground

---

Second option: if you want to go with more specific to match exact 2 fields name and text both just need to add $let operator to return fields from filter,

db.collection.find({
  "input.fields": { $elemMatch: { "name": "last_name_hebrew" } },
  "output.fields": { $elemMatch: { "name": "last_name_hebrew" } },
  $expr: {
    $eq: [
      {
        $let: {
          vars: {
            input: {
              $arrayElemAt: [
                {
                  $filter: {
                    input: "$input.fields",
                    cond: { $eq: ["$$this.name", "last_name_hebrew"] }
                  }
                },
                0
              ]
            }
          },
          in: { name: "$$input.name", text: "$$input.text" }
        }
      },
      {
        $let: {
          vars: {
            output: {
              $arrayElemAt: [
                {
                  $filter: {
                    input: "$output.fields",
                    cond: { $eq: ["$$this.name", "last_name_hebrew"] }
                  }
                },
                0
              ]
            }
          },
          in: { name: "$$output.name", text: "$$output.text" }
        }
      }
    ]
  }
})

Playground

---

Third option: for more specific to check both fields in loop,

• first filter the matching elements by name in input field using $filter
• pass above filter result in another filter
• filter to match name and text field in output field, if its not [] empty then return filter result
• $ne to check return result is not [] empty

db.collection.find({
  "input.fields": { $elemMatch: { "name": "last_name_hebrew" } },
  "output.fields": { $elemMatch: { "name": "last_name_hebrew" } },
  $expr: {
    $ne: [
      {
        $filter: {
          input: {
            $filter: {
              input: "$input.fields",
              cond: { $eq: ["$$this.name", "last_name_hebrew"] }
            }
          },
          as: "i",
          cond: {
            $ne: [
              {
                $filter: {
                  input: "$output.fields",
                  cond: {
                    $and: [
                      { $eq: ["$$this.name", "$$i.name"] },
                      { $eq: ["$$this.text", "$$i.text"] }
                    ]
                  }
                }
              },
              []
            ]
          }
        }
      },
      []
    ]
  }
})

Playground