Sometimes I see variable names with double underscore in the beginning and the end. For example:
Article.__elasticsearch__
Is there some naming convention related to double underscores in Ruby variable names?
An initial underscore or double underscore basically indicates "special/avoid overwrite" --meaning it's meant to reduce the likelihood that someone else might define a method/attribute of the same name. The most common occurrence is
__send__.
From Ruby Forum
The author of the ElasticSearch gem made the wrong call IMO. At the end of the thread, Avdi Grimm, who is well-known in the Ruby community, disagrees with the OP.
There's a reason you hadn't seen it yet and that it looks odd to you. It's because it's unidiomatic.
来源：https://stackoverflow.com/questions/23270937/ruby-variable-name-with-double-underscores