Question

mongoDB怎么返回数组字段数组的指定索引的

Answer 1

MongoDB 2.2's new $elemMatch projection operator provides another way to alter the returned document to contain only the first matched shapes element:

db.test.find(
{"shapes.color": "red"},
{_id: 0, shapes: {$elemMatch: {color: "red"}}});
Returns:

{"shapes" : [{"shape": "circle", "color": "red"}]}
In 2.2 you can also do this using the $
projection operator, where the $ in a projection object field name represents the index of the field's first matching array element from the query. The following returns the same results as above:

db.test.find({"shapes.color": "red"}, {_id: 0, 'shapes.$': 1});
mongodb 3.2 Update

Starting with the 3.2 release, you can use the new $filter aggregation operator to filter an array during projection, which has the benefit of including all matches, instead of just the first one.

db.test.aggregate([
// Get just the docs that contain a shapes element where color is 'red'
{$match: {'shapes.color': 'red'}},
{$project: {
shapes: {$filter: {
input: '$shapes',
as: 'shape',
cond: {$eq: ['$$shape.color', 'red']}
}},
_id: 0
}}
])
Results:

[
{
"shapes" : [
{
"shape" : "circle",
"color" : "red"
}
]
}
]