I have collection which has an array field as follows.
availableProducts: [ "mobile", "laptop", "desktop"]
Now I would like to update this field to a string field at the same time i need to keep the values of array as comma separated strings as follows
availableProducts: "mobile,laptop,desktop"
Can we do this with almost 1 million records in MongoDB version 4.0
Much easier and understandable way:
db.collection.aggregate([
{
$addFields: {
"availableProducts": {
$reduce: {
input: "$availableProducts",
initialValue: "",
in:{
$concat:[
"$$value",
{
"$cond":{
"if":{
"$eq":[
"$$value",
""
]
},
"then":"",
"else":","
}
},
"$$this"
]
}
}
}
}
},
{$out:'collection'}
])
Source: https://docs.mongodb.com/manual/reference/operator/aggregation/reduce/
You can do this from mongo shell, as follows:
Input documents:
{ "_id" : 1, "a" : [ "blue", "green", "red" ] }
{ "_id" : 2, "a" : [ "cat", "dog", "rat" ] }
The query:
db.arr1.find()
.forEach( doc => { doc.a = doc.a.toString(); db.arr1.save(doc); } )
Result (the updated collection):
{ "_id" : 1, "a" : "blue,green,red" }
{ "_id" : 2, "a" : "cat,dog,rat" }
You can achieve this with the following aggregation :
db.collection.aggregate([
{
$addFields: {
"availableProducts": {
$reduce: {
input: "$availableProducts",
initialValue: "",
in: {
"$cond": {
if: {
"$eq": [
{
"$indexOfArray": [
"$availableProducts",
"$$this"
]
},
0
]
},
then: {
"$concat": [
"$$value",
"$$this"
]
},
else: {
"$concat": [
"$$value",
",",
"$$this"
]
}
}
}
}
}
}
},
{$out:'collection'}
])
Use it carefully, as the $out stage will replace your entire collection with the result. My advice is to test without $out stage before.
You can test it here
