edits and rearranging logic

docs/howto/passing_arbitrary_options.md

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+[Back to Guides](../README.md)
+
+# Passing Arbitrary Options To A Serializer
+
+In addition to the [`serialization_scope`](../general/serializers.md#scope), any options passed to `render`
+that are not reserved for the [adapter](../general/rendering.md#adapter_opts)
+are available in the serializer as [instance_options](../general/serializers.md#instance_options).
+
+For example, we could pass in a field, such as `user_id` into our serializer.
+
+```ruby
+# posts_controller.rb
+class PostsController < ApplicationController
+  def dashboard  
+    render json: @post, user_id: 12
+  end
+end
+
+# post_serializer.rb
+class PostSerializer < ActiveModel::Serializer
+  attributes :id, :title, :body
+
+  def comments_by_me  
+    Comments.where(user_id: instance_options[:user_id], post_id: object.id)
+  end
+end
+```
+
+Since `user_id` isn't a reserved adapter option, we can process it via a serializer
+method. The option is passed via the `instance_options` hash.