如何获取集合 MongoDB 中多个文档的单个字段值？

Question

I have a collection of documents with the following schema:

{ 
    "_id" : ObjectId("8dcaac2eb104c2133d66f144"), 
    "Shape" : "circle",  
    "Color" : "blue" 
},

My goal is to fetch the value of a specific field dynamically for a range of documents. A response to a request of the Color value of 3 documents should look like this:

{
    "blue"
    "green"
    "yellow"
}

I'm using Mongodb and nodejs, here's my code:

var field = req.params.field
var field_option = {};
field_option[field] = 1;

db.collection.find({_id: {$gte: first, $lt: last}}, field_option).toArray(function(err, data){
    if(err || !data) throw err;
    res.json(data);
});

However, the output is wrong:

[
    {
        "_id": "8dcaac2eb104c2133d66f144",
        "Shape" : "circle",  
        "Color" : "blue" 
    },
    {
        "_id": "8dcaac2eb104c2133d66f145",
        "Shape" : "square",  
        "Color" : "green" 
    },
    {
        "_id": "8dcaac2eb104c2133d66f146",
        "Shape" : "triangle",  
        "Color" : "yellow" 
    }

]

Answer 1

To get this as a response you have to manipulate the results after getting the data from mongo query.

var data = res.json(data), endResult = [];
for(key in data) {
    endResult[key] = data[key]["Color"];
}
return endResult; 

Answer 2

The object you want is wrong

{
  "blue"
  "green"
  "yellow"
}

You need manage the result in a array.

[
  "blu",
  "green",
  "yellow",
]

There 2 way using a agregate or manipulating the result

1 way:

db.collection.aggregate([
  { $match: { _id: {$gte: first, $lt: last} }},
  { $group: { _id: "1", fields: { $push: { $concat: field }}}}
]);

it returns a object with the positions _id, and fields with the result. Because the pipeline of the aggregation do the match and after group by a defautl value and put into a array the field you want

2 way: Manipulating the result using a map.

db.collection.find({_id: {$gte: first, $lt: last}}, field_option).toArray(function(err, data){
    if(err || !data) throw err;
    res.json(data.map(item => (item[field]);
});

It returns the array you want, data is a array of object, and the function map is used to reestructure the object to a string.

The second way is easier than the first and impact less the code.