Use a setting value from one MongoDB collection in a lookup referece in another collection aggregration

Lets' say I have a collection called settings, that has a document where the _name field has a value of A and in there there is a data.year value i.e.

"settings" : [
 { 
    _id: ObjectId('123123123123'),
    _name: "A",
    data: {
      year: 2025
    }
 }
]

I am then creating an aggregation in another collection, let's call it B, where my lookup is matching documents in A but needs to be filtered based on the year value from the settings collection document above i.e.

"B": [
  { 
    shortName: "B1"
  },
  {
    shortName: "B2"
  }
]

"A" : [
  {
    name: "A2",
    shortNameInA: "B1",
    datesInfo: {
     closed: ISODate("2025-12-10"),
    }
  },
  {
    name: "A3",
    shortNameInA: "B2",
    datesInfo: {
     report: ISODate("2025-03-10"),
    }
  },
  {
    name: "A1",
    shortNameInA: "B1",
    datesInfo: {
     closed: ISODate("2024-12-10"),
    }
  }
]

db.B.aggregate([
{
  from: "A",
  localField: "shortName",
  foreignField: "shortNameInA",
  pipeline: [
    {
      $match: {
        $expr: {
          $or: [
            {
              $eq: [
                {
                  $year: "$datesInfo.closed"
                },
                {
                  $year: "$$NOW"   <--- This needs to be `year` value from settings
                }
              ]
            },
            {
              $eq: [
                {
                  $year:
                    "$datesInfo.report"
                },
                {
                  $year: "$$NOW"  <--- This needs to be `year` value from settings
                }
              ]
            }
          ]
        }
      }
    }
  ],
  as: "_fromA"
}

What I need to do is pass in that year value instead of $$NOW - how do I do that ?

So, in this example it would pass in 2025 and I should get the following view:

[
  {   
    shortName: "B1",
    _fromA: [
     {
       name: "A2",
       shortNameInA: "B1",
       datesInfo: {
         closed: ISODate("2025-12-10"),
       }
     }
    ]
  },
  {
    shortName: "B2",
    _fromA: [
      {
        name: "A3",
        shortNameInA: "B2",
        datesInfo: {
          report: ISODate("2025-03-10"),
        }
      }
    ]
  }    
]

A full example is available here.

You can do a separate, unconditional$lookup to the settings collection first. Then, continue with your current $lookup by setting data.year as a variable

db.B.aggregate([
  {
    "$lookup": {
      "from": "settings",
      "let": {
        "settingsKey": "A"
      },
      "pipeline": [
        {
          "$match": {
            $expr: {
              $eq: [
                "$$settingsKey",
                "$_name"
              ]
            }
          }
        }
      ],
      "as": "settingsLookup"
    }
  },
  {
    "$unwind": "$settingsLookup"
  },
  {
    $lookup: {
      from: "A",
      localField: "shortName",
      foreignField: "shortNameInA",
      let: {
        settingYear: "$settingsLookup.data.year"
      },
      pipeline: [
        {
          $match: {
            $expr: {
              $or: [
                {
                  $eq: [
                    {
                      $year: "$datesInfo.closed"
                    },
                    "$$settingYear"
                  ]
                },
                {
                  $eq: [
                    {
                      $year: "$datesInfo.report"
                    },
                    "$$settingYear"
                  ]
                }
              ]
            }
          }
        }
      ],
      as: "_fromA"
    }
  },
  {
    "$unset": "settingsLookup"
  }
])

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