Ruby ActiveRecord 对象的减号运算符

Question

你能解释一下吗，Ruby 的减号运算符是如何工作的？不仅仅是像 5 - 2 这样的小案例。有点复杂——我们有 2 个带有 ActiveRecord 对象的数组：数组 A = User.where(...)，数组 B = User.where(...)，我想制作 A - B，它是如何工作的？它只是比较对象 ID 或所有属性，还是其他？

Answer 1

Is it just comparing objects IDs?

类型和 ID，是的。

pry(main)> show-source User#eql?

From: /Users/sergio/.gem/ruby/2.5.1/gems/activerecord-5.2.0/lib/active_record/core.rb @ line 420:
Owner: ActiveRecord::Core
Visibility: public
Number of lines: 6

def ==(comparison_object)
  super ||
    comparison_object.instance_of?(self.class) &&
    !id.nil? &&
    comparison_object.id == id
end

Answer 2

Sergio 回答的更多细节：

我缩小了 - 方法（运算符）对 ActiveRecord::Relation 对象的作用，因为我自己也很好奇：

Rails 5:

回溯：

# rails console (pry-rails)
users_a = User.where(...)
users_b = User.where(...)

puts users_a.class
# => `User::ActiveRecord_Relation`

show-source users_a.-

# From: /Users/jrpolidario/.rbenv/versions/2.4.1/lib/ruby/gems/2.4.0/gems/activerecord-5.2.0/lib/active_record/relation/delegation.rb @ line 41:
# Owner: ActiveRecord::Delegation
# Visibility: public
# Number of lines: 4
#
# delegate :to_xml, :encode_with, :length, :each, :uniq, :join,
#          :[], :&, :|, :+, :-, :sample, :reverse, :rotate, :compact, :in_groups, :in_groups_of,
#          :to_sentence, :to_formatted_s, :as_json,
#          :shuffle, :split, :slice, :index, :rindex, to: :records

# since the `-` method as seen above is delegated to #records, let's see what the return type is the return value of `#records` is, of which is supposed to respond to the `-` operator.

puts users_a.records.class
# => Array

# ...because it's an Array type, then let's see if the Array type responds to the delegated `-` method.

show-source users_a.records.-

# From: array.c (C Method):
# Owner: Array
# Visibility: public
# Number of lines: 17
# 
# static VALUE
# rb_ary_diff(VALUE ary1, VALUE ary2)
# {
#     VALUE ary3;
#     VALUE hash;
#     long i;
# 
#     hash = ary_make_hash(to_ary(ary2));
#     ary3 = rb_ary_new();
# 
#     for (i=0; i<RARRAY_LEN(ary1); i++) {
#   if (st_lookup(rb_hash_tbl_raw(hash), RARRAY_AREF(ary1, i), 0)) continue;
#   rb_ary_push(ary3, rb_ary_elt(ary1, i));
#     }
#     ary_recycle_hash(hash);
#     return ary3;
# }

... 这只是简单的意思，我引用 Array

Returns a new array that is a copy of the original array, removing any items that also appear in other_ary. The order is preserved from the original array.

Rails 4

P.S。我也尝试在 rails 4.2 中跟踪这个，并且 show-source users_a.- 没有显示任何方法，这意味着它使用了 method_missing （因此也意味着在 4 和 5 之间有变化关于这个），然后，进一步追踪，我得到以下结果：

回溯：

[127, 136] in /Users/jrpolidario/.rbenv/versions/2.5.1/lib/ruby/gems/2.5.0/gems/activerecord-4.2.10/lib/active_record/relation/delegation.rb
   127: 
   128:     def method_missing(method, *args, &block)
   129:       if @klass.respond_to?(method)
   130:         scoping { @klass.public_send(method, *args, &block) }
   131:       elsif array_delegable?(method)
=> 132:         to_a.public_send(method, *args, &block)
   133:       elsif arel.respond_to?(method)
   134:         arel.public_send(method, *args, &block)
   135:       else
   136:         super

... 如您所见，.to_a 在 ActiveRecord::Relation 对象上被调用，这意味着它成为一个数组，然后 method（即 '-') 在该 Array 对象上被调用，这也意味着最后它也会调用上面 Rails 5 中相同的 Array#- 方法。

Ruby ActiveRecord 对象的减号运算符

Ruby minus operator with ActiveRecord objects

ruby

benchmarking

activerecord

ruby-on-rails

Rails 5:

回溯：

Rails 4

回溯：