db={"items":[{category:"A"}],"partners":[{_id:"supportedPartner",supportedCategories:[{category:"A",active:true}]},{_id:"unsupportedPartner1",supportedCategories:[{category:"B",active:true}]},{_id:"unsupportedPartner2",supportedCategories:[{category:"A",active:false}]}]}

db.items.aggregate([{"$match":{"category":"A"}},{"$lookup":{"from":"partners","let":{"mycat":"$category"},"pipeline":[{"$match":{"$expr":{"$anyElementTrue":{"$map":{"input":"$supportedCategories","as":"cat","in":{$and:[{$eq:["$$cat.category","$$mycat"]},{$eq:[true,"$$cat.active"]}]}}}}}}],as:"joinedPartners"}}])