MySQL 查询在 Rails 3.2.11 的 postgresql 中不起作用

Question

我在 Rails 中使用以下查询

grouped_link_counters = uattachment).link_counters.group("year(created_at), month(created_at)").count("created_at")

它使用 mysql 适配器在 Rails 控制台中给出以下结果。

  UserAttachment Load (0.2ms)  SELECT `user_attachments`.* FROM `user_attachments` WHERE `user_attachments`.`id` = 132 LIMIT 1
   (0.2ms)  SELECT COUNT(`link_counters`.`created_at`) AS count_created_at, year(created_at), month(created_at) AS year_created_at_month_created_at FROM `link_counters` WHERE `link_counters`.`user_attachment_id` = 132 GROUP BY year(created_at), month(created_at)
 => {11=>9, 12=>15, 1=>1, 2=>1} 

但是当我使用 postgresql 时，相同的命令会抛出以下错误:-

SELECT COUNT("link_counters"."created_at") AS count_created_at, year(created_at), month(created_at) AS year_created_at_month_created_at FROM "link_counters" WHERE "link_counters"."user_attachment_id" = 1 GROUP BY year(created_at), month(created_at)
ActiveRecord::StatementInvalid: PG::Error: ERROR:  function year(timestamp without time zone) does not exist
LINE 1: ...link_counters"."created_at") AS count_created_at, year(creat...
                                                             ^
HINT:  No function matches the given name and argument types. You might need to add explicit type casts.
: SELECT COUNT("link_counters"."created_at") AS count_created_at, year(created_at), month(created_at) AS year_created_at_month_created_at FROM "link_counters"  WHERE "link_counters"."user_attachment_id" = 1 GROUP BY year(created_at), month(created_at)
    from /home/nishutosh/.rvm/gems/ruby-1.9.3-p194@cdrop/gems/activerecord-3.2.11/lib/active_record/connection_adapters/postgresql_adapter.rb:1153:in `async_exec'
    from /home/nishutosh/.rvm/gems/ruby-1.9.3-p194@cdrop/gems/activerecord-3.2.11/lib/active_record/connection_adapters/postgresql_adapter.rb:1153:in `exec_no_cache'
    from /home/nishutosh/.rvm/gems/ruby-1.9.3-p194@cdrop/gems/activerecord-3.2.11/lib/active_record/connection_adapters/postgresql_adapter.rb:662:in `block in exec_query'
    from /home/nishutosh/.rvm/gems/ruby-1.9.3-p194@cdrop/gems/activerecord-3.2.11/lib/active_record/connection_adapters/abstract_adapter.rb:280:in `block in log'
    from /home/nishutosh/.rvm/gems/ruby-1.9.3-p194@cdrop/gems/activesupport-3.2.11/lib/active_support/notifications/instrumenter.rb:20:in `instrument'
    from /home/nishutosh/.rvm/gems/ruby-1.9.3-p194@cdrop/gems/activerecord-3.2.11/lib/active_record/connection_adapters/abstract_adapter.rb:275:in `log'
    from /home/nishutosh/.rvm/gems/ruby-1.9.3-p194@cdrop/gems/activerecord-3.2.11/lib/active_record/connection_adapters/postgresql_adapter.rb:661:in `exec_query'
    from /home/nishutosh/.rvm/gems/ruby-1.9.3-p194@cdrop/gems/activerecord-3.2.11/lib/active_record/connection_adapters/postgresql_adapter.rb:1248:in `select'
    from /home/nishutosh/.rvm/gems/ruby-1.9.3-p194@cdrop/gems/activerecord-3.2.11/lib/active_record/connection_adapters/abstract/database_statements.rb:18:in `select_all'
    from /home/nishutosh/.rvm/gems/ruby-1.9.3-p194@cdrop/gems/activerecord-3.2.11/lib/active_record/connection_adapters/abstract/query_cache.rb:63:in `select_all'
    from /home/nishutosh/.rvm/gems/ruby-1.9.3-p194@cdrop/gems/activerecord-3.2.11/lib/active_record/relation/calculations.rb:289:in `execute_grouped_calculation'
    from /home/nishutosh/.rvm/gems/ruby-1.9.3-p194@cdrop/gems/activerecord-3.2.11/lib/active_record/relation/calculations.rb:206:in `perform_calculation'
    from /home/nishutosh/.rvm/gems/ruby-1.9.3-p194@cdrop/gems/activerecord-3.2.11/lib/active_record/relation/calculations.rb:159:in `calculate'
    from /home/nishutosh/.rvm/gems/ruby-1.9.3-p194@cdrop/gems/activerecord-3.2.11/lib/active_record/relation/calculations.rb:58:in `count'
    from (irb):63
    from /home/nishutosh/.rvm/gems/ruby-1.9.3-p194@cdrop/gems/railties-3.2.11/lib/rails/commands/console.rb:47:in `start'
    from /home/nishutosh/.rvm/gems/ruby-1.9.3-p194@cdrop/gems/railties-3.2.11/lib/rails/commands/console.rb:8:in `start'
    from /home/nishutosh/.rvm/gems/ruby-1.9.3-p194@cdrop/gems/railties-3.2.11/lib/rails/commands.rb:41:in `<top (required)>'
    from script/rails:6:in `require'
    from script/rails:6:in `<main>'1.9.3p194 :064 >

任何人都可以告诉我如何才能从 postgresql 获得相同的响应。提前致谢!

Answer 1

在 PostgreSQL 中，我们没有像 YEAR、MONTH 等辅助函数。您可以在 PostgreSQL 中使用 date_part 来代替这些函数与 MySQL 的结果相同。

例如

grouped_link_counters = uattachment.link_counters.group("date_part('year', created_at), date_part('month', created_at)").count("created_at")