Ways to verify if two items within a collection of objects share a common value in MongoDB

I have a collection of data called users stored in mongoDB that has the following structure:

_id: ObjectId,
sports: [
  {
    name: 'cricket',
    history: [
      {
        from: 10,
        to: 30
      },
      {
        from: 30,
        to: 30
      }
    ]
  },
  // ... other sports as well
]

My goal is to query for users who have at least one element inside sports.history where the condition from === to is true. Each user can have multiple sports, each with its own history data.

I'm looking to achieve this filtering directly within the query, rather than fetching users and then applying the filter in my express app afterwards.

Any assistance on how to accomplish this would be greatly appreciated. Thank you!

javascriptmongodbexpressmongoose

Answer №1

Utilizing the $expr operator, you can effectively query the collection by flattening the 2D array '$sports.history' and filtering it based on certain conditions using array operators. Start by flattening the array with $reduce:

 {
    $reduce: {
        input: "$sports.history",
        initialValue: [],
        in: { $concatArrays: [ "$$value", "$$this" ] }
    }
}

Then, apply a filter to the reduced array using $filter

{ $filter: {
    input: {
        $reduce: {
            input: "$sports.history",
            initialValue: [],
            in: { $concatArrays: [ "$$value", "$$this" ]
            }
        }
    },
    cond: {
        $eq: ['$$this.from', '$$this.to']
    }
} }

Determine the length of the resulting array using $size:

{ $size: { 
    { $filter: {
        input: {
            $reduce: {
                input: '$sports.history',
                initialValue: [],
                in: { $concatArrays: [ '$$value', '$$this' ] }
            }
        },
        cond: {
            $eq: ['$$this.from', '$$this.to']
        }
    } }
} }

If the length of the filtered array is greater than zero, then the user exists:

{ $gt: [
    { $size: { 
        $filter: {
            input: {
                $reduce: {
                    input: "$sports.history",
                    initialValue: [],
                    in: { $concatArrays: [ "$$value", "$$this" ] }
                }
            },
            cond: {
                $eq: ['$$this.from', '$$this.to']
            }
        }
    } },
    0
] }

In conclusion, your final query should resemble this:

db.users.find({
    $expr: {
        $gt: [
            { $size: {
                $filter: {
                    input: {
                        $reduce: {
                            input: "$sports.history",
                            initialValue: [],
                            in: { $concatArrays: [ "$$value", "$$this" ] }
                        }
                    },
                    cond: {
                        $eq: ['$$this.from', '$$this.to']
                    }
                }
            } },
            0
        ]
    }
})

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