MongoDB 在事先不知道所有字段的情况下聚合字段 [英] MongoDB aggregate fields without knowing all the fields before hand
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问题描述
How can I compute the aggregate of the following metrics without knowing all the metrics before hand? Can I accomplish this using the aggregate framework or MapReduce?
[
{
player_id: '123',
timestamp: <sometime>,
metrics: {
points_per_game: 1,
rebounds_per_game: 2,
assist_per_game: 3,
}
},
{
player_id: '123',
timestamp: <sometime>,
metrics: {
points_per_game: 1,
rebounds_per_game: 2,
}
},
{
player_id: '345',
timestamp: <sometime>,
metrics: {
points_per_game: 1,
rebounds_per_game: 2,
point_in_the_paint_per_game: 2
}
}
]
I would like to have the following result
[
{
player_id: '123',
metrics: {
points_per_game: 2,
rebounds_per_game: 4,
assist_per_game: 3,
}
},
{
player_id: '345',
metrics: {
points_per_game: 1,
rebounds_per_game: 2,
point_in_the_paint_per_game: 2
}
}
]
I cannot do something like the following since it would require me to know every metrics:
db.stats.aggregate([
{$group: {
_id: {player: "$player_id"},
points_per_game: { $sum: "$metrics.points_per_game"}
...
])
解决方案
You can try below aggregation.
Convert the object into array of key value pairs followed by $unwind+$group to group by each key and accumulate the count. Final step to go back to named key value object.
db.colname.aggregate([
{"$addFields":{"metrics":{"$objectToArray":"$metrics"}}},
{"$unwind":"$metrics"},
{"$group":{
"_id":{"id":"$player_id","key":"$metrics.k"},
"count":{"$sum":"$metrics.v"}
}},
{"$group":{
"_id":"$_id.id",
"metrics":{"$mergeObjects":{"$arrayToObject":[[["$_id.key","$count"]]]}}
}}
])
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