问题
Help to "flatten" (to pull nested fields at same level as document's fields) a mongodb document in a query
//this is "anagrafiche" collection
[{
"name": "tizio"
,"surname": "semproni"
,"birthday": "01/02/1923"
,"home": {
"road": "via"
,"roadname": "bianca"
,"roadN": 12
,"city": "rome"
,"country": "italy"
}
},
{
"name": "caio"
,"surname": "giulio"
,"birthday": "02/03/1932"
,"home": {
"road": "via"
,"roadname": "rossa"
,"roadN": 21
,"city": "milan"
,"country": "italy"
}
},
{
"name": "mario"
,"surname": "rossi"
// birthday is not present for this document
,"home": {
"road": "via"
,"roadname": "della pace"
,"roadN": 120
,"city": "rome"
,"country": "italy"
}
}
]
my query:
db.anagrafiche.aggregate([ {$match {"home.city": "rome"}}
{$project:{"name": 1, "surname":1, <an expression to flatten the address>, "birthday": 1, "_id":0}}
]
);
expected result:
{
,"name": "tizio"
,"surname": "semproni"
,"address": "via bianca 12 rome"
,"birthday": 01/02/1923
},{
,"name": "mario"
,"surname": "rossi"
,"address": "via della pace 120 rome"
,"birthday": NULL
}
回答1:
You can use $objectToArray to get nested document keys and values and then use $reduce along with $concat to concatenate values dynamically:
db.collection.aggregate([
{
$project: {
_id: 0,
name: 1,
surname: 1,
birthday: 1,
address: {
$reduce: {
input: { $objectToArray: "$home" },
initialValue: "",
in: {
$concat: [
"$$value",
{ $cond: [ { $eq: [ "$$value", "" ] }, "", " " ] },
{ $toString: "$$this.v" }
]
}
}
}
}
}
])
Mongo Playground
来源:https://stackoverflow.com/questions/62346889/mongodb-how-to-flatten-some-query-results