Explain hash rocket in this context

Question

I've just written this code, that works although I am not entirely sure why:

scope = scope.where(Sequel.qualify(:meeting_graphs, :id) => query.ids)

I am specifically talking about the hash rocket.

Previously the code was this, which makes perfect sense:

scope = scope.where(id: query.ids)

First thing I do not understand is why it does this not work when I replace the hash rocket with a colon which I thought was the preferred syntax:

scope = scope.where(Sequel.qualify(:meeting_graphs, :id): query.ids)

Sequel.qualify returns an object which also confuses me as I thought it would return a symbol.

Can anyone explain?

Marek Lipka · Answer 1 · 2013-10-03T10:36:31.117

3

New hash syntax works only if key is a literal symbol.
Sequel.qualify returns qualifier object identifying column. It's possible since every object can be a hash key in Ruby.

edited Oct 03 '13 at 10:36

answered Oct 03 '13 at 08:54

Marek Lipka

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I would say "a literal symbol" – Sergio Tulentsev Oct 03 '13 at 08:56
Sequel.qualify returns an object – dagda1 Oct 03 '13 at 10:02
@dagda1 virtually everything in Ruby is object. What kind of object does `Sequel.qualify` return? – Marek Lipka Oct 03 '13 at 10:03
A QualifiedObject http://sequel.rubyforge.org/rdoc/classes/Sequel/SQL/QualifyingMethods.html – dagda1 Oct 03 '13 at 10:34

score 1 · Accepted Answer · answered Oct 03 '13 at 09:00

that works although I am not entirely sure why

As long as Sequel.qualify(:meeting_graphs, :id) is valid, it can be a key of a hash. Any object can be a key of a hash. That is why.
why it does this not work when I replace the hash rocket with a colon

Even if Sequel.qualify(:meeting_graphs, :id) turns out to be a symbol, the colon notation will not work because it is part of a literal notation. It is not a method or a keyword that works on Ruby objects that are already made.

score 0 · Answer 3 · edited May 23 '17 at 10:32

You are passing a keywords to the function and keywords use Hash syntax.

There are many ways to define hashes in ruby, and in the way you use a function one syntax does not work.

def return_one_symbol
   'one'.to_sym
end

hash_syntax1 = {:one => '1'}
hash_syntax2 = {one: '1'}
p hash_syntax1 # => {:one=>"1"}
p hash_syntax2 # => {:one=>"1"}

hash_syntax1_function = {return_one_symbol => '1'}
hash_syntax2_function = {return_one_symbol: '1'}
p hash_syntax1_function # => {:one=>"1"}
p hash_syntax2_function # => {:return_one_symbol=>"1"}

see this post for more info:

Is there any difference between the `:key => "value"` and `key: "value"` hash notations?

Explain hash rocket in this context

3 Answers3