ActiveRecord中Find与Where区别

find： 根据id查找，返回一个对象
find_by_*： 返回一个对象
find_all_by_*：返回Array
where： 返回ActiveRecord::Relation，操作对象时才执行SQL

在下面的示例中你可以清楚地比较各个语句的返回值和SQL

>> Bookmark.find(1)
SELECT "bookmarks".* FROM "bookmarks" WHERE "bookmarks"."id" = ? LIMIT 1  [["id", 1]]
#

>> Bookmark.find(1).class
SELECT "bookmarks".* FROM "bookmarks" WHERE "bookmarks"."id" = ? LIMIT 1  [["id", 1]]
Bookmark(id: integer, name: string, order: integer, url: string)

>> Bookmark.find_by_order(0).class
SELECT "bookmarks".* FROM "bookmarks" WHERE "bookmarks"."order" = 0 LIMIT 1
Bookmark(id: integer, name: string, order: integer, url: string)

>> Bookmark.find_all_by_order(0).class
SELECT "bookmarks".* FROM "bookmarks" WHERE "bookmarks"."order" = 0
Array

>> Bookmark.where(:order => 0).class
#不执行SQL
ActiveRecord::Relation

>> Bookmark.where(:order => 0)
SELECT "bookmarks".* FROM "bookmarks" WHERE "bookmarks"."order" = 0
[#,
 #]

>> Bookmark.where(:order => 0).first.class
SELECT "bookmarks".* FROM "bookmarks" WHERE "bookmarks"."order" = 0 LIMIT 1
Bookmark(id: integer, name: string, order: integer, url: string)