Question

如何在ActiveRecord中每个唯一列值获取2条或更多记录？

给出：

Name     | School
Jonathan | Sunshine College 
Dylan    | Sunshine College
Matt     | Sunshine College
Joseph   | Sunshine College
Stephen  | Greenville School
Phil     | Greenville School
Warren   | Greenville School  
JohnPaul | Greenville School

然后，如果我想每所学校输出 2条记录 ，它将看起来像这样：

Name     | School
Jonathan | Sunshine College 
Dylan    | Sunshine College
Stephen  | Greenville School
Phil     | Greenville School

如果我希望每所学校 3条记录 ，那么它将如下所示：

Name     | School
Jonathan | Sunshine College 
Dylan    | Sunshine College
Matt     | Sunshine College
Stephen  | Greenville School
Phil     | Greenville School
Warren   | Greenville School

Answer 1

您可以按所需的列值对表的数据进行分区。之后，您可以使用条件来判断每个分区要多少条记录：

Student
  .from(
    Student.select('*, row_number() OVER (PARTITION BY school) AS rownum')
  )
  .select('*')
  .where('rownum < 2')

Answer 2

这是不使用sql的一种方法。

Student.group(:school)
   .each{|student| Student.where(school: student.school)
   .first(2)
   .each{|result| puts "#{result.name.ljust(20, ' ')}| #{result.school}"}}

# Phil                | Greenville School
# Warren              | Greenville School
# Jonathan            | Sunshine College
# Dylan               | Sunshine College

Answer 3

将模型的集合转换为可以比较相等性的属性列表时，可以uniq：

MyModel.all.pluck(:first_name, :last_name).uniq

如何在ActiveRecord中每个唯一列值获取2个或更多记录？

3 个答案: