I'm new to Ruby on Rails and I couldn't understand the reason. Can you help me? I'm new to Ruby on Rails and I couldn't understand the reason. What does '<=>' mean in Ruby on Rails? The problem is: I'm setting up a component structure using phlex in Rails dynamically. My product is full when initialized with prod_col, but when I do div(class: @comp.props['_class']) { @product.send(@comp.props['source']) }, I get an error."
def initialize(comp, product:)
@comp = comp
@product = product
end
def template
if @comp._folder == 'prod_catalog'
plain(@product.send(@comp.props['source']))
else
div(class: @comp.props['_class']) { @product.send(@comp.props['source']) }
end
end
I'm actually sending the column of the products in the database, and I want to get the unit name or unit names of the relevant product as a return. However, I encounter this error.
Most probably your issue is not really about the method <=> (yes, that is astonishingly a method call though) - but that you try to call it on the object nil. Which ruby version do you use? On 3.3.1 the method <=> is not private on nil NilClass. But if I remember correctly, that differed before.
If you want to understand the method <=> you can check its source code. I like to start a pry console for something alike:
[2] pry(main)> require 'pry-doc'
=> true
[3] pry(main)> show-source nil.<=>
From: object.c (C Method):
Owner: Kernel
Visibility: public
Signature: <=>(arg1)
Number of lines: 7
static VALUE
rb_obj_cmp(VALUE obj1, VALUE obj2)
{
if (rb_equal(obj1, obj2))
return INT2FIX(0);
return Qnil;
}
[4] pry(main)> nil <=> nil
=> 0
[5] pry(main)> nil <=> 2
=> nil
[6] pry(main)> nil <=> 'test'
=> nil
[7] pry(main)> nil.<=> 'test'
=> nil
Just to get a feeling, what this kind of is about. The method <=> normally is used to describe a comparision operator. That is expected to return 0 if both objects are equal. It returns -1 if first object is smaller and +1 if the argument (so the second object) is smaller. So just a small hint about it with some more code about it:
[8] pry(main)> 1 <=> 2
=> -1
[9] pry(main)> 2 <=> 1
=> 1
[10] pry(main)> show-source 1.<=>
From: numeric.c (C Method):
Owner: Integer
Visibility: public
Signature: <=>(arg1)
Number of lines: 13
VALUE
rb_int_cmp(VALUE x, VALUE y)
{
if (FIXNUM_P(x)) {
return fix_cmp(x, y);
}
else if (RB_BIGNUM_TYPE_P(x)) {
return rb_big_cmp(x, y);
}
else {
rb_raise(rb_eNotImpError, "need to define `<=>' in %s", rb_obj_classname(x));
}
}
I really hope, that your @comp does not include some user input. Else this @product.send request is highly dangerous.
In the end I could not really answer about your real problem, since you missed to give some more context. But maybe this helps already about your question: "What does '<=>' mean in Ruby on Rails?"
I hope I can give you a little more insights into the method <=> which is used behind the scenes i. e. if you sort an array, with following small playing around on console:
[17] pry(main)> class Test
[17] pry(main)* def initialize(a,b)
[17] pry(main)* @a, @b = a, b
[17] pry(main)* end
[17] pry(main)* def <=>(test2)
[17] pry(main)* return 0 if @a == test2.a && @b == test2.b
[17] pry(main)* return -1 if @a <= test2.a && @b <= test2.b
[17] pry(main)* return 1 if @a >= test2.a && @b >= test2.b
[17] pry(main)* nil # you could remove this line, since it's default, if all the guards before fail - I just wanted to make the default return value obvious here
[17] pry(main)* end
[17] pry(main)* attr_reader :a, :b
[17] pry(main)* end
[18] pry(main)> [Test.new(6,7), Test.new(1,2), Test.new(4,5)].sort
=> [#<Test:0x00007f9741fb1c80 @a=1, @b=2>, #<Test:0x00007f9741fb1c30 @a=4, @b=5>, #<Test:0x00007f9741fb1cd0 @a=6, @b=7>]
[19] pry(main)> [Test.new(1,2), Test.new(4,5), Test.new(1,7)].sort
ArgumentError: comparison of Test with Test failed
from (pry):36:in `sort'
The second sort breaks, since Test.new(1,7) is not comparable to the other two instances in our <=> definition (returns nil then, instead of -1, 0 or 1).