[英]arel, how to join
特定
class Category < ActiveRecord::Base
has_many :products, :order => 'name ASC'
end
使用Rails 3堆栈,如何查询“拥有”产品的所有类别?
在ARel(非ActiveRecord)中,我们将执行以下操作:
p = Arel::Table.new :products # Base Rel-var
c = Arel::Table.new :categories # Base Rel-var
predicate = p[:category_id].eq( c[:id] ) # for equality predicate
p.join(c) # Natural join
.on( predicate ) # Equi-Join
.group( p[:category_id] ) # Grouping expression to get distinct categories
.project( c[:id] ) # Project the distinct category IDs of the derived set.
Category.joins(:products).select("distinct categories.*").all
另一种更简单的方法是将ActiveRecord查询接口的join
与ARel一起用于条件语句:
joins(:user)
.where(User.arel_table[:name].matches("%#{query}%"))
在sqlite3中生成以下sql:
"SELECT \"patients\".* FROM \"patients\" INNER JOIN \"users\" ON \"users\".\"id\" = \"patients\".\"user_id\" WHERE (\"users\".\"name\" LIKE '%query%')"
和postgres中的以下sql(注意ILIKE):
"SELECT \"patients\".* FROM \"patients\" INNER JOIN \"users\" ON \"users\".\"id\" = \"patients\".\"user_id\" WHERE (\"users\".\"name\" ILIKE '%query%')"
这允许您简单地加入,但仍然可以将ARel匹配器抽象到您的RDBMS。
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