Filtering at create - rails - ruby-on-rails

Question

I'm trying to check if new submissions match certain aspects of existing submissions and, if so, prevent it from being created.
if ( !Book.exists?(author: #book.author) and
!Book.exists?(publisher: #book.publisher) )
or
( !Book.exists?(name: #book.name) and
!Book.exists?(genre: #book.genre) )
...create
The problem is that if the genre and the publisher match existing records, the book is not created. That's clearly not what I intend with those operators. I tried && and || and also mixed them with and and or, knowing && and || take precedence. I also tried placing the second logic into elsif. No use. I'd appreciate any help.
UPDATE:
I have this code now in the model. I deleted if #book.save in the controller. When I save, the page does nothing, but the button remains frozen at clicked.
validate :existing_book, on: :create
# also tried before_create :existing_book
def existing_book
existing_book = Book.find_by(author: self.author, publisher: self.publisher)
existing_book ||= Book.find_by(name: self.name, genre: self.genre)
if existing_book.nil?
self.save
redirect_to book_url(self)
else
throw(:abort)
redirect_to new_book_url(self)
errors.add("matching record exists")
end
end
I added throw(:abort) after reading that in Rails 5+, returning false doesn't abort the process (it didn't when I tried it).
Error message: No template found for BooksController#create, rendering head :no_content. So I'm guessing #book.save should be in the controller? But before_create still saves
--UPDATE:
I re-added .save in the controller:
if #book.save
redirect_to book_url(#book)
else
redirect_to new_book_url(#book), alert: "Please try again."
end
The model:
before_create :existing_book
# validate :existing_book, on: :create
def existing_book
existing_book = Book.find_by(author: self.author, publisher: self.publisher)
existing_book ||= Book.find_by(name: self.name, genre: self.genre)
if existing_book != nil
return false
end
end
The record still gets created. Same when before_save instead of before_create.

Answer 1

Your validate method should not call save, redirect, or raise any error. Those are handled by the controller code. The model-level validate should either add errors to the instance, or do nothing at all.
For example (and simplifying the code a bit to focus on the concept):
validate :existing_book
def existing_book
existing_book = Book.find_by(name: self.name)
if existing_book != nil
errors.add(:name, "already taken")
end
end
And then testing it out:
existing_name = Book.first.name
new_book = Book.new(name: existing_name)
new_book.valid? # false
new_book.errors.full_messages # => ["name already taken"]
new_book.save # false
In the controller, for example:
book = Book.new(book_params)
if book.save
# redirect to page on success
else
#errors = book.errors.full_messages
# render the `new` page, showing the errors
end

Answer 2

Add a before_create filter to your model. Let's suppose you name it check_existing_records.
before_create :check_existing_records # add this line to the top of your model
You want to add code inside the body of your method which will return false in case it is detected that the new instance should not be saved. Something like:
def check_existing_records
existing_book = Book.find_by(author: self.author, publisher: self.publisher)
# in case no book with same author and publisher was found, check for book
# with same name and same genre.
existing_book ||= Book.find_by(name: self.name, genre: self.genre)
# returns false if there is an existing record which matches these conditions
# and stops persistence of record in the database
existing_book.nil?
end

Answer 3

You can use the find_or_create_by method for this:
book = Book.find_or_create_by(author: #book.author, publisher: #book.publisher)
This command will find any existing book with the matching author and publisher or created a new book record if there is no such entry.