Question 257689 in Mongodb Question posted in
The official documentation can be found here.

Mongodb – Create query to unwind array and filter out elements after an element with a certain value

I have a document like this in a collection:

{ 
  _id: '123', 
  locations: [ 
    {uuid:'1-1'}, 
    {uuid:'2-2'}, 
    {uuid:'3-3'}, 
    {uuid:'4-4'} 
  ] 
}

I would like to create a query that returns this:

[
  { location: { uuid:'3-3'} }
  { location: { uuid:'4-4'} }
]

Looks like a job for aggregate

Start off by:

{ $match: { _id:  123 } }

then maybe

{ $unwind: '$locations' }

then

{ $project: { _id: 0, location: '$locations'} }

Cool, this gives me:

[
  { location: { uuid: '1-1'} }
  { location: { uuid: '2-2'} }
  { location: { uuid: '3-3'} }
  { location: { uuid: '4-4'} }
]

Now i need a way to only return elements that come after ‘3-3’ (but also keep 3-3). Any ideas?

Tags:

2

Answers


  1. 0

    Here the approach I took is to $slice the array from index 2 to end of array so to get the position of 3-3 i used $indexOfArray.
    Now since locations is sliced there is fewer to $unwind

    db.collection.aggregate([
  { $match: { _id: "123" } },
  {
    $project: {
      _id: 0,
      locations: {
        $slice: [ "$locations", 
            // If target is not found, $indexOfArray will return -1.
            // OP says in that case, return whole array which means
            // start at 0.
            {$max: [0, { $indexOfArray: [ "$locations", { uuid: "3-3" } ] } ] },
            { $size: "$locations" } ]
        }
    }
  },
  { $unwind: "$locations" },
  { $project: { _id: 0, location: "$locations" } }
])

    demo

    Login or Signup to reply.
  2. 0

    The accepted answer works but for extra protection, you might want to do $indexOfArray on the uuid target, not the object:

    {$max: [0,{ $indexOfArray: [ "$locations.uuid", "3-3"] }] },

    This ensures correct matching on only uuid. For example, assume the input objects are more complex and possibly polymorphic:

    { 
  _id: '123', 
  locations: [ 
    {uuid:'1-1', other:1}, 
    {uuid:'2-2', foo:2}, 
    {uuid:'3-3', baz:3, not_concerned:7}, 
    {uuid:'4-4', corn:4} 
  ] 
}

    Then the original approach:

    { $indexOfArray: [ "$locations", { uuid: "3-3" } ] }

    will fail because of additional fields baz and not_concerned.

    Login or Signup to reply.
Please signup or login to give your own answer.

Back To Top
Search