问题
I have array of parent documents consisting of nested documents ranging between 20 to 500. How can I limit the number of nested documents shown for each parent document.
below is the structure of my Document and nested document.
[{
id
title
users: [{
user_id: 1,
timestamp: 2354218,
field3: 4
}, {
user_id: 1,
timestamp: 2354218,
field3: 4
}, {
user_id: 1,
timestamp: 2354218,
field3: 4
},
...
]
}, {
},
...
]
I want to limit the number of users shown for each parent document. how to?
My Query
db.movies.aggregate(
[{$match: {
"movie_title": "Toy Story (1995)"}
},{
$lookup: {
from: "users",
localField: "users.user_id",
foreignField: "users.id",
as: "users"
}
},
{$project: {
movie_title: "$movie_title",
users: { $slice: [ "$users", 1 ] }
}}
]);
回答1:
You can try below query. Use $slice to get at most first n elements in nested documents array for each document.
db.collection.aggregate([{ $project: { title: 1, nUsers: { $slice: [ "$users", n ] } } ])
or Using regular query.
db.collection.find({}, { title: 1, nUsers: {$slice: n } })
来源：https://stackoverflow.com/questions/42258325/how-to-limit-the-number-of-nested-documents-shown-in-mongodb