[Avram] What is the correct way to retrieve one record from db, and invoke methods on it.

[zw963]

We assume users table, exists a open_id column can be retreive the unique record, but it can be nil if open_id is not correct.

Solution 1: works

open_id = "open_id"
user : User = UserQuery.new.open_id(open_id).first # this may raise exception.

user.account
user.password_digest

It works, but, first maybe raise exception instead if no matched record, so we should do rescue, right?

Solution 2 works

open_id = "open_id"
user = UserQuery.new.open_id(open_id).first? # it should be type User | Nil

return nil if user.blank?

# return nil if user.locked_at  # this line code not work, because `undefined method 'locked_at' for Nil`

user.try &.locked_at

But, all method invoke on user must use &, it so tediously.

Solution 3 not work

user : User | Nil

user_query = UserQuery.new.open_id(open_id)

if user_query.any?
  user  = user_query.first
end

here, user type is User | Nil, i have user_query.any? if condition, why still this?

So, my question is, what is the correct way to do those things in varam? i am still newbie to crystal and lucky, if there is a another recommended why to do this?

[jwoertink]

In Crystal, you can assign the nilable value to a variable within the if condition, and the compiler will know the type.

if user = UserQuery.new.open_id("open_id").first?
  # this works because Crystal compiler knows `user` exists here.
  user.account
end

You can also do something like

if user_query.any?
  user = user_query.first.as(User)
end

The .as(User) will tell the compiler that the object is that type if a Union is possible (User | Nil).

[Blacksmoke16]

def some_method
  open_id = "open_id"

  return unless (user = UserQuery.new.open_id(open_id).first?)

  user # : User
end

[robacarp]

Yes, crystal doesn't trust @vars types against guards. You can easily assign it to a local:

class Foo
  getter bar : String?
  def initialize
  end

  def print_bar
    if @bar.nil?
      # @bar is still String?
    else
      # @bar is still String?
    end

    bar = @bar
    if bar.nil?
      # bar is known to be a Nil
    else
      # bar is known to be a String
    end
  end
end

[zw963]

@grepsedawk

It appears like user might be a getter for a @user ivar?

no, this is a variable, if it is a getter, sure this usage is invalid.

https://crystal-lang.org/reference/1.4/syntax_and_semantics/if_var.html#method-calls

[zw963]

There is a difference between #nil? (which is a compiler psuedo-method) and #blank? which is a method declared on String. #nil? informs the compiler and influences the code after it, while #blank? doesn't.

Cool, you point out the key!!

Yes, my code works after change blank? to nil?, you are awesome!

[zw963]

  return unless (user = UserQuery.new.open_id(open_id).first?)

This code works too, thank you, it equivalent to:

user = UserQuery.new.open_id("open_id").first?

    return nil unless user

    return nil if user.locked_at  # work!

[zw963]

#1700 (reply in thread) is consider as the expected answer.

Thanks all!