How to select custom data in mongo - mongodb

Question

Is there a way how to include custom data in the mongo query response?
What I mean is a mongo alternative for something like this in MySQL code:
SELECT
value,
'7' AS min_value
FORM
my_table
WHERE
value >= 7
...while the 7 should probably be a variable in the language where the mongo query is being called from.

Answer 1

Try the $literal operator if using the aggregation framework with a $match pipeline step as your query filter. For example, create a sample collection in mongo shell that has 10 test documents with the value field as an increasing integer (0 to 9):
for(x=0;x<10;x++){ db.my_table.insert({value: x }) }
Running the following aggregation pipeline:
var base = 7;
db.my_table.aggregate([
{
"$match": {
"value": { "$gte": base }
}
},
{
"$project": {
"value": 1,
"min_value": { "$literal": base }
}
}
])
would produce the result:
/* 0 */
{
"result" : [
{
"_id" : ObjectId("561e2bcc3d8f561c1548d39b"),
"value" : 7,
"min_value" : 7
},
{
"_id" : ObjectId("561e2bcc3d8f561c1548d39c"),
"value" : 8,
"min_value" : 7
},
{
"_id" : ObjectId("561e2bcc3d8f561c1548d39d"),
"value" : 9,
"min_value" : 7
}
],
"ok" : 1
}

Answer 2

The only things in MongoDB query actions that actuallly "modify" the results returned other than the original document or "field selection" are the .aggregate() method or the JavaScript manipulation alternate in mapReduce.
Otherwise documents are returned "as is", or at least with just the selected fields or array entry specified.
So if you want something else returned from the server, then you need to use one of those methods:
var seven = 7;
db.collection.aggregate([
{ "$match": {
"value": { "$gt": seven }
}},
{ "$project": {
"value": 1,
"min_value": { "$literal": seven }
}}
])
Where the $literal operator comes into play, or in versions prior to 2.6 and greater or equal to 2.2 ( aggregation framework introduced ) can use $const instead:
var seven = 7;
db.collection.aggregate([
{ "$match": {
"value": { "$gt": seven }
}},
{ "$project": {
"value": 1,
"min_value": { "$const": seven }
}}
])
Or just use mapReduce and it's JavaScript translation:
var seven = 7;
db.mapReduce(
function() {
emit(this._id,{ "value": this.value, "min_value": seven });
},
function() {}, // no reduce at all since all _id unique
{
"out": { "inline": 1 },
"query": { "value": { "$gt": seven } },
"scope": { "seven": seven }
}
);
Those are basically your options.