[
  { "_id":1,    "make":"Honda",    "model":"Accord"  },
  { "_id":2,    "make":"Toyota",   "model":"Camry"  },
  { "_id":3,    "make":"Honda",    "model":"Civic"  },
  { "_id":4,    "make":"Subaru",   "model":"Legacy"  },
  { "_id":5,    "make":"Subaru",   "model":"Outback"  },
  { "_id":6,    "make":"Toyota",   "model":"Corrola"  }
]

[
    {   "description" : "Honda" },
    {   "description" : "Honda - Accord"    },
    {   "description" : "Honda - Civic"     },
    {   "description" : "Subaru"    },
    {   "description" : "Subaru - Legacy"   },
    {   "description" : "Subaru - Outback"  },
    {   "description" : "Toyota"    },
    {   "description" : "Toyota - Camry"    },
    {   "description" : "Toyota - Corrola"  }
]

db.cars.aggregate([
   { $sort : { make : 1, model:1 } },
   { $project: {"_id":0, "description":{ $concat: [ "$make", " - ", "$model" ]} } }
])

[
    {   "description" : "Honda - Accord"    },
    {   "description" : "Honda - Civic"     },
    {   "description" : "Subaru - Legacy"   },
    {   "description" : "Subaru - Outback"  },
    {   "description" : "Toyota - Camry"    },
    {   "description" : "Toyota - Corrola"  }
]

db.cars.aggregate([
    { $sort: { make: 1, model: 1 }},
    { $group: {
            "_id": "$make",
            models: {
                $push: {
                    $concat: ["$make", " - ", "$model"]
                }
            }
        }
    },
    { $project: {
            "_id": 0,
            "description": {
                $reduce: {
                    input: "$models",
                    initialValue: "$_id",
                    in: {
                        $concatArrays: ["$$value", "$$this"]
                    }
                }
            }
        }
    }
])

"errmsg" : "$concatArrays only supports arrays, not string",
"code" : 28664,