在导轨中，给定Point对象的数组，我如何求和每个玩家的积分值？

Question

We're using postgres for this project.

The goal is to get a hash which pairs each player with the sum of their point values.

I have an array of Point objects which looks something like this:

( Note : each hash here is actually a database object from Points.where . The array is actually an ActiveRecord:Relation instance)

[
    { value: 3, player_id: 55 },
    { value: 1, player_id: 21 },
    { value: 2, player_id: 55 },
    { value: 6, player_id: 23 },
    { value: 2, player_id: 78 },
    { value: 2, player_id: 55 },
    { value: 5, player_id: 80 },
    { value: 7, player_id: 21 },
    { value: 2, player_id: 23 },
    { value: 1, player_id: 78 },
    { value: 4, player_id: 80 }
]

The expected output of this would be something like this:

{
    55: 7,
    21: 8,
    23: 8,
    78: 3,
    80: 9
}

Where the key is the player_id, and the value is the sum of all of their point values.

One solution in ruby I came up with uses activerecord to sum the points for each ID in the set.

points = get_the_points()

point_map = {}
ids = points.pluck(:player_id)

player_ids.each do |id|
    points_for_id = points.where(player_id: id).pluck(:value)
    sum_of_points = points_for_id.sum

    point_map[id] = sum_of_points
end

return point_map

But I know this can be done with SQL, and I know it'll be faster.

Because I'm just dealing with one model, I don't have to use joins or anything, do I?

Answer 1

尝试一个SQL查询

Point.group(:player_id).sum(:value)

Answer 2

In plain SQL we would want something like:

SELECT points.user_id, SUM(points.value) AS sum_value
FROM points
GROUP BY points.user_id;

Which groups the rows by user_id and fetches an aggregate of points.value .

 user_id | sum_value 
---------+-----------
      14 |        18
      25 |        19
      17 |        29
      12 |        14
       8 |         9
       1 |        16
      15 |        19
      10 |        37
      11 |        26
       4 |        43

To get the same query in Rails you would use:

Point.group(:user_id).sum(:value)

Because I'm just dealing with one model, I don't have to use joins or anything, do I?

Not unless you want to do something like a high score list where you list the users name and score.