在Rails中，如何按数组字段中元素的个数进行分组？

Question

给定 ActiveRecord 模型 Foo，其字段 a_of_i 是整数数组：

# Foo:
id, a_of_i
101, [1,2,3,4]
102, []
103, [11, 12, 13, 14, 15, 16]
104,  [1, 2, 52, 53]

是否有一个 ActiveRecord 查询将按数组列中#elements 的计数进行分组，例如 produce:

# Foo.some_query returns something like [[count, id], [count, id]...]
=> [ [6, 103], [4, 101], [4, 104], [0, 102] ]
# or perhaps, if easier:
=> { 6 => [103], 4 => [101,104], 0 => [102] }

Answer 1

剧透

Foo.select('id, cardinality(a_of_i)').pluck('cardinality(a_of_i)', :id)

创建 table

CREATE TABLE foos(
  id   text,
  a_of_i integer[]
);

在ruby中填写table

Foo.create id: 101, a_of_i: [1,2,3,4]
Foo.create id: 102, a_of_i: []
Foo.create id: 103, a_of_i: [11, 12, 13, 14, 15, 16]
Foo.create id: 104, a_of_i: [1, 2, 52, 53]

思考sql

SELECT 
  id,
  CARDINALITY( a_of_i )
FROM
  foos

有 AR

Foo.select('id, cardinality(a_of_i)').pluck('cardinality(a_of_i)', :id)

Returns

[[4, 101], [0, 102], [6, 103], [4, 104]]

在Rails中，如何按数组字段中元素的个数进行分组？

In Rails, how to group by count of elements in array field?

arrays

activerecord

ruby-on-rails