Ruby on Rails MYSQL查询

Question

I am pretty inexperienced with MySQL queries, but I am currently learning them. How would I find all the nil fields of an object and then populate just one of those fields?

For example, I have an inventory - but I want to find all the empty slots to show me how many slots I have left and then find ones that are empty, and then to populate just one of those fields with a string or number.

How would I do this?

This is the code I attempted:

@cast = Cart.where("user_id = ? AND cart_slot_one = nil AND cart_slot_two = nil AND cart_slot_three = nil AND cart_slot_four = ? AND cart_slot_five = ? AND cart_slot_six = ? AND cart_slot_seven = ? AND cart_slot_eight = ? AND cart_slot_nine = ? AND cart_slot_ten = ?",
    current_user.id, nil, nil, nil, nil, nil, nil, nil, nil, nil, nil ).first

Answer 1

To find a card in which all slots are empty :

slots = %w( one two three four five six seven eight nine ten )
query = Hash[slots.map { |slot| ["cart_slot_#{slot}", nil] }]

@cart = Cart.where(query).find_by(user: current_user)

Or, if your user has an association to carts:

@cart = current_user.slots.find_by(query)

To find a cart with at least one empty slot :

slots = %w( one two three four five six seven eight nine ten )
query = slots.map { |slot| "cart_slot_#{slot} IS NULL"] }.join(' OR ')

@cart = Cart.where(query).find_by(user: current_user)

Once you find such a cart use the following lines to count or determine empty slots:

# count slots:
count = slots.count { |slot| @cart.send("cart_slot_#{slot}").nil? }

# get first empty slot:
slot_number = slots.find { |slot| @cart.send("cart_slot_#{slot}").nil? }

Answer 2

@cast = Cart.where(user: current_user, cart_slot_one: nil, cart_slot_two: nil, cart_slot_three: nil, cart_slot_four: nil, cart_slot_five: nil, cart_slot_six: nil, cart_slot_seven: nil, cart_slot_eight: nil)
@cast.each do |c|
  c.update_attributes(cart_slot_one: foo') #update all the attributes you want here
end

Answer 3

1. Instead of .where(...).first you can use .find_by(...) which is basically the same but implicitly returns the first element or nil
2. Consider using composition instead of hard-coding slots. You could have a class CartSlot and in the Cart has_many :cart_slots. Instead of checking each on are empty you could check if there are any.

3. To improve your solution:

    class Cart def self.find_first_empty_for_user(current_user_id) find_by(user_id: current_user_id, cart_slot_one: nil, cart_slot_two: nil, cart_slot_three: nil, cart_slot_four: nil, cart_slot_five: nil, cart_slot_six: nil, cart_slot_seven:nil, cart_slot_eight: nil, cart_slot_nine: nil, cart_slot_ten: nil) end end def process(current_user, new_value = 'Whatever) cart = Cart.find_first_empty_for_user(current_user.try(:id)) return unless empty_cart cart.cart_slot_one = new_value cart.save! cart end