ArgumentError（传递给#or的关系必须在结构上兼容。不兼容的值：[:select]）

Question

这行得通...

a = User.select("users.*, 1 as car_id").limit(2)
b = User.select("users.*, 1 as car_id").limit(2)
a.or(b)
>> returns a combined AR Relation

然而，

a = User.select("users.*, 1 as car_id").limit(2)
b =  User.select("users.*, 2 as car_id").limit(2)
a.or(b)
>> ArgumentError (Relation passed to #or must be structurally compatible. Incompatible values: [:select])

如果我做a+b，它会合并，然后转换为array，我想保持AR关系以继续进行查询。

当select的car_id值不同时，不能union。我原以为相同的列名会允许联合。

我正在模型上创建虚拟属性 (car_id)，因为每组记录都需要使用不同的值定义 car_id。

如何解决错误并与虚拟属性联合？

我想要一个 ActiveRecord 关系，并且足够有趣：

SELECT users.*, 1 as car_id
FROM users
UNION
SELECT users.*, 2 as car_id
FROM users

在运行原始 sql 时工作正常。

SQL 示例：

 a.mail_for(:inbox).name

=> Mailboxer::Conversation::ActiveRecord_Relation

 a.mail_for(:inbox).to_sql

"SELECT DISTINCT mailboxer_conversations.*, '102' as mailer_id, 'Listing' as mailer_type FROM \"mailboxer_conversations\" INNER JOIN \"mailboxer_notifications\" ON \"mailboxer_notifications\".\"conversation_id\" = \"mailboxer_conversations\".\"id\" AND \"mailboxer_notifications\".\"type\" IN ('Mailboxer::Message') INNER JOIN \"mailboxer_receipts\" ON \"mailboxer_receipts\".\"notification_id\" = \"mailboxer_notifications\".\"id\" WHERE \"mailboxer_notifications\".\"type\" = 'Mailboxer::Message' AND \"mailboxer_receipts\".\"receiver_id\" = 102 AND \"mailboxer_receipts\".\"receiver_type\" = 'Listing' AND \"mailboxer_receipts\".\"mailbox_type\" = 'inbox' AND \"mailboxer_receipts\".\"trashed\" = FALSE AND \"mailboxer_receipts\".\"deleted\" = FALSE ORDER BY \"mailboxer_conversations\".\"updated_at\" DESC" 

所以我将一系列关系传递给 union_scope...

 ar= a.mail_for(:inbox)
 br= b.mail_for(:inbox)
 cr= c.mail_for(:inbox)

combined = union_scope([[a,ar],[b, br],[c, cr])

def union_scope(*relation)
      combined = relation.first[1].none
      relation.each do |relation_set|
        mailer = relation_set[0]
        scope = relation_set[1].select("#{relation_set[1].table_name}.*, \'#{mailer.id}\' as mailer_id, \'#{mailer.class.name}\' as mailer_type")        
        combined = combined.or(scope)
      end
      combined
    end

更新：

def union_scope(*relation)
  combined = relation.first[1].none
  relation.each do |relation_set|
    mailer = relation_set[0]
    scope = relation_set[1].select("#{relation_set[1].table_name}.*, \'#{mailer.id}\' as mailer_id, \'#{mailer.class.name}\' as mailer_type")        
    combined = combined.union(scope)
  end
  conv = ::Mailboxer::Conversation.arel_table
  ::Mailboxer::Conversation.from(conv.create_table_alias(combined, :conversations).to_sql)
end

导致了这个错误：

Mailboxer::Conversation Load (5.8ms)  SELECT "mailboxer_conversations".* FROM ( SELECT DISTINCT "mailboxer_conversations".* FROM "mailboxer_conversations" INNER JOIN "mailboxer_notifications" ON "mailboxer_notifications"."conversation_id" = "mailboxer_conversations"."id" AND "mailboxer_notifications"."type" IN ('Mailboxer::Message') INNER JOIN "mailboxer_receipts" ON "mailboxer_receipts"."notification_id" = "mailboxer_notifications"."id" WHERE "mailboxer_notifications"."type" = $1 AND "mailboxer_receipts"."receiver_id" = $2 AND "mailboxer_receipts"."receiver_type" = $3 AND (1=0) ORDER BY "mailboxer_conversations"."updated_at" DESC UNION SELECT DISTINCT mailboxer_conversations.*, '102' as mailer_id, 'Listing' as mailer_type FROM "mailboxer_conversations" INNER JOIN "mailboxer_notifications" ON "mailboxer_notifications"."conversation_id" = "mailboxer_conversations"."id" AND "mailboxer_notifications"."type" IN ('Mailboxer::Message') INNER JOIN "mailboxer_receipts" ON "mailboxer_receipts"."notification_id" = "mailboxer_notifications"."id" WHERE "mailboxer_notifications"."type" = $4 AND "mailboxer_receipts"."receiver_id" = $5 AND "mailboxer_receipts"."receiver_type" = $6 ORDER BY "mailboxer_conversations"."updated_at" DESC ) "conversations"

ActiveRecord::StatementInvalid (PG::SyntaxError: ERROR:  syntax error at or near "UNION")
LINE 1: ...ER BY "mailboxer_conversations"."updated_at" DESC UNION SELE...
                                                             ^
: SELECT "mailboxer_conversations".* FROM ( SELECT DISTINCT "mailboxer_conversations".* FROM "mailboxer_conversations" INNER JOIN "mailboxer_notifications" ON "mailboxer_notifications"."conversation_id" = "mailboxer_conversations"."id" AND "mailboxer_notifications"."type" IN ('Mailboxer::Message') INNER JOIN "mailboxer_receipts" ON "mailboxer_receipts"."notification_id" = "mailboxer_notifications"."id" WHERE "mailboxer_notifications"."type" = $1 AND "mailboxer_receipts"."receiver_id" = $2 AND "mailboxer_receipts"."receiver_type" = $3 AND (1=0) ORDER BY "mailboxer_conversations"."updated_at" DESC UNION SELECT DISTINCT mailboxer_conversations.*, '102' as mailer_id, 'Listing' as mailer_type FROM "mailboxer_conversations" INNER JOIN "mailboxer_notifications" ON "mailboxer_notifications"."conversation_id" = "mailboxer_conversations"."id" AND "mailboxer_notifications"."type" IN ('Mailboxer::Message') INNER JOIN "mailboxer_receipts" ON "mailboxer_receipts"."notification_id" = "mailboxer_notifications"."id" WHERE "mailboxer_notifications"."type" = $4 AND "mailboxer_receipts"."receiver_id" = $5 AND "mailboxer_receipts"."receiver_type" = $6 ORDER BY "mailboxer_conversations"."updated_at" DESC ) "conversations"

Answer 1

虽然我无法回答您关于为什么这些不兼容的实际问题，但我可以提供类似于您的原始 SQL 示例的解决方案。

你可以像这样执行相同的操作

user_table = User.arel_table
a = user_table.project(Arel.star, Arel.sql("1 as car_id")).take(2)
b = user_table.project(Arel.star, Arel.sql("2 as car_id")).take(2)
union = Arel::Nodes::UnionAll.new(a,b)
User.from(Arel::Nodes::As.new(union,user_table)) 

这将导致以下查询。

SELECT 
  users.* 
FROM 
  ( (SELECT  *, 1 as car_id 
     FROM users  
     ORDER BY 
       users.id ASC 
     LIMIT 2) UNION ALL (
     SELECT  *, 2 as car_id 
     FROM users  
     ORDER BY 
       users.id ASC 
     LIMIT 2)) AS users

由于这仍会返回 ActiveRecord::Relation，因此您仍然可以像对其他任何对象一样操作此对象，我们只需将普通数据源（用户表）替换为同名的自定义联合数据源。

基于极端修订的更新和一些关于你实际打算做什么的一般假设

a= a.mail_for(:inbox)
b= b.mail_for(:inbox)
c= c.mail_for(:inbox)


combined = union_scope(a,b,c)

def union_scope(*relations)
  base = build_scope(*relations.shift)
  combined = relations.reduce(base) do |memo, relation_set|
    Arel::Nodes::UnionAll.new(memo,build_scope(*relation_set))
  end
  union = Arel::Nodes::As.new(combined,::Mailboxer::Conversation.arel_table)
  ::Mailboxer::Conversation.from(union)
end
def build_scope(mailer,relation)
   relation.select(
      "#{relation.table_name}.*, 
      '#{mailer.id}' as mailer_id, 
      '#{mailer.class.name}' as mailer_type"
   ).arel 
end