預先加載的Rails 3的ActiveRecord模型的基表條件

Question

我想要得到這樣的結果： 被公佈，有一個特定的標記，並且包括標籤的所有帖子（避免N + 1）

這裏是我的代碼：

@posts = Post.includes(
    :tags 
).where(
    :status => 'published', :tags => { :name => params[:tag_name] } 
).order(
    'published_at DESC' 
) 

這裏是軌道的輸出：

Parameters: {"tag_name"=>"family"} Post Load (1.1ms) SELECT "posts"."id" AS t0_r0, "posts"."title" AS t0_r1, "posts"."body" AS t0_r2, "posts"."published_at" AS t0_r3, "posts"."excerpt" AS t0_r4, "posts"."slug" AS t0_r5, "posts"."created_at" AS t0_r6, "posts"."updated_at" AS t0_r7, "posts"."status" AS t0_r8, "tags"."id" AS t1_r0, "tags"."name" AS t1_r1, "tags"."created_at" AS t1_r2, "tags"."updated_at" AS t1_r3 FROM "posts" LEFT OUTER JOIN "posts_tags" ON "posts_tags"."post_id" = "posts"."id" LEFT OUTER JOIN "tags" ON "tags"."id" = "posts_tags"."tag_id" WHERE "posts"."status" = 'published' AND "tags"."name" = 'family' ORDER BY published_at DESC Completed in 102ms

以下是錯誤消息：

/home/sam/.rvm/gems/ruby-1.8.7-p334@global/gems/activerecord-3.0.6/lib/active_record/attribute_methods.rb:44:in `eval': missing attribute: status 

我可以從該posts.status列化名爲t0_r8的SQL看，但我怎麼能得到正常的尊重我的條件？

編輯 查詢我有，這不工作：

@posts = Post.joins(
    :tags 
).where(
    "posts.status = ? AND tags.name = ?", 
    "published", 
    params[:tag_name] 
).order(
    "published_at DESC" 
) 

Answer 1

嗯，也許我只是不完全明白ActiveRecord的工作方式;這是我想出瞭解決方案：

@posts = Post.includes(
    :tags 
).joins(
    :tags 
).where(
    "posts.status = ? AND tags.name LIKE ?", 
    "published", 
    params[:tag_name] 
).order(
    "published_at DESC" 
).paginate(
    :per_page => 5, 
    :page => params[:page], 
    :order => "published_at DESC" 
) 

我的錯誤是在假設joins和includes將複製的功能，顯然不是：

Post Load (1.7ms) SELECT "posts".* FROM "posts" INNER JOIN "posts_tags" ON "posts_tags"."post_id" = "posts"."id" INNER JOIN "tags" ON "tags"."id" = "posts_tags"."tag_id" WHERE (posts.status = 'published' AND tags.name LIKE 'family') ORDER BY published_at DESC Tag Load (1.1ms) SELECT "tags".*, t0.post_id as the_parent_record_id FROM "tags" INNER JOIN "posts_tags" t0 ON "tags".id = t0.tag_id WHERE (t0.post_id IN (7,6,4))