MongoDB : Use $unwind into $lookup pipeline

Question

I currently need to perform a query with MongoDB using the aggregation framework. Here is the diagram of my documents as well as the request I wish to make.

tools

{
    "_id": 1,
    "code": "TOOL_1",
    "label": "Tool 1",
    "items": [
        {
            "code_it": TOOL_1_TT_1,
            "label": "Tool TT 1",
            "value": 25.00
        },
        {
            "code_it": TOOL_1_TT_2,
            "label": "Tool TT 2",
            "value": 17.00
        },
        {
            "code_it": TOOL_1_TT_3,
            "label": "Tool TT 3",
            "value": 17.00
        },
    ]
}

results_tools

{
    "_id": 1,
    results: [
        {
            "code_it": "TOOL_TT_2",
            "owner": "person_A"
        },
        {
            "code_it": "TOOL_TT_3",
            "owner": "person_D"
        },
        {
            "code_it": "TOOL_TT_2",
            "owner": "person_C"
        },
    ]
}


db.results_tools.aggregate([

    { $unwind: "$results" },
    { 
        $lookup: {
            from: "tools",
            let: { res_cod_it: "$code_it" },
            pipeline: [
                { $unwind: "$items" },
                { 
                    $match: { 
                        $expr: { 
                            $eq: [ "$$res_cod_it", "$code_it" ] 
                        }
                    }
                }
            ],
            as: "item"
        }
    },
    {
        $project: {
            "owner": 1,
            "el.code": "code_it",
            "el.label": "item.label",
            "el.value": "item.label"
        }
    }

]);

I would like to be able to retrieve the label of the tool based on the results. However, I have an error at runtime, so I wonder if using $unwind in the $lookup pipeline is possible.

Thank you for your answer. Laurent

Answer 1

You can use below pipeline. Note the use of dot notation to reference the array fields.

db.results_tools.aggregate([
  {"$unwind":"$results"},
  {"$lookup":{
    "from":"tools",
    "let":{"res_cod_it":"$results.code_it"},
    "pipeline":[
      {"$unwind":"$items"},
      {"$match":{"$expr":{"$eq":["$$res_cod_it","$items.code_it"]}}}
    ],
    "as":"item"
  }},
  {"$unwind":"$item"},
  {"$project":{
    "owner":"$results.owner",
    "code":"$item.items.code_it",
    "label":"$item.items.label",
    "value":"$item.items.value"
  }}
])