[Rails / ActiveRecord] I want to validate the value before the type is converted (_before_type_cast)


In ActiveRecord, if you put a character string in a Date type or Integer type column, it will be automatically converted to that column type.

For example, if you have the following users table: (Created_at and updated_at are omitted for simplicity.)

   create_table "users", force: :cascade do |t|
    t.string "name"
    t.integer "age"

Let's try entering various values using the Rails console.

 irb (main): 001: 0> User.new (name: "Taro", age: 21) Try putting a number in #age
 => # <User name: "Taro", age: 21>

 irb (main): 002: 0> User.new (name: "Taro", age: "21") Try putting a character string in #age
 => # <User name: "Taro", age: 21>

 irb (main): 003: 0> User.new (name: "Taro", age: "21") Try putting a character string in #age (1 is a full-width number)
 => # <User name: "Taro", age: 2>

The first is a numerical value. Since it is a numerical value, it can be entered as it is.

The second is a string. The string "21" has been converted to the number 21. Wonderful.

How about the third one, the 1 part of "21" is a full-width number. It has become age: 2. It's completely different from 21.

By the way, even if you use "2 1" (with a space between them) or "2 a" (with a character other than a number), it will be age: 2.

What's wrong

Suppose you try to play a non-numeric input and apply the following validation.

  validates :age, format: { with: /\A[0-9]+\z/}

In this case, if the user inputs "2 1", "2 a", etc., validation will be applied to 2 of age, and it will be saved as correct data.

Solution "\ _before_type_cast"

The solution is to add _before_type_cast to age to validate the value entered by the user instead of the value after the type has been converted. [* Reference "before_type_cast with rails / validates"](https://dora.bk.tsukuba.ac.jp/~takeuchi/?%E3%82%BD%E3%83%95%E3%83%88%E3 % 82% A6% E3% 82% A7% E3% 82% A2% 2Frails% 2Fvalidates% E3% 81% A7before_type_cast)

  validates :age_before_type_cast, format: { with: /\A[0-9]+\z/ }, presence: true

Applications (such as string conversion)

In addition to validation, _before_type_cast is also useful when you want to convert alphanumeric characters in a character string from full-width to half-width, or when you want to save after removing spaces.

As an example, the following code uses a gem called Moji to convert from full-width to half-width.

This time, I use _before_type_cast because I want to align the input that is a mixture of full-width and half-width such as "21" (1 is full-width) of the user to half-width before validation.

After receiving the character string as it is input by the user with age_before_type_cast and converting it, the converted character string is put in age.

  validates :age_before_type_cast, format: { with: /\A[0-9]+\z/ }, presence: true

  before_validation do
    string = self.send(:age_before_type_cast)
    string = Moji.normalize_zen_han(string) 
    send(:write_attribute, :age, string)

The nature of _before_type_cast

Difference between 〇〇 and 〇〇_before_type_cast

First is the difference between normal attributes and attributes with _before_type_cast. The value cannot be obtained with [: 〇〇_before_type_cast]. In particular

 irb (main): 001: 0> user = User.new (name: "Taro", age: "21")

irb(main):002:0> user.age
=> 21
irb(main):003:0> user.age_before_typecast
=> "21"

irb(main):004:0> user[:age] 
=> 21
irb(main):005:0> user[:age_before_typecast] 
=> nil 

You can get the value with user.age_before_type_cast, When I try to get the value using user [: age_before_type_cast], nil is returned.

〇〇_before_type_cast cannot be updated

irb(main):001:0> user.age = 21
=> 21
irb(main):002:0> user.age_before_type_cast = 21
 => #NoMethodError occurs.

Since _before_type_cast is for checking the input before type conversion to age, of course it is not possible to update itself.

What happens after save

Also, if you save, 〇〇 and 〇〇_before_type_cast will be the same. In particular,

 irb (main): 001: 0> user = User.new (name: "Taro", age: "21")
irb(main):002:0> user.save
irb(main):003:0> user.age_before_typecast
=> 21

The number 21 is returned instead of the string "21".

Be careful when implementing or testing because of the above properties!

Reference article

[before_type_cast with rails / validates](https://dora.bk.tsukuba.ac.jp/~takeuchi/?%E3%82%BD%E3%83%95%E3%83%88%E3%82%A6 % E3% 82% A7% E3% 82% A2% 2Frails% 2Fvalidates% E3% 81% A7before_type_cast)

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