from django.db import models
from django.urls import reverse 
import uuid # Required for unique book instances


class Genre(models.Model):
    """
    Model representing a book genre (e.g. Science Fiction, Non Fiction).
    """
    name = models.CharField(max_length=200, 
        help_text="Enter a book genre (e.g. Science Fiction, French Poetry etc.)")


    def __str__(self):
        """
        String for representing the Model object (in Admin site etc.)
        """
        return self.name


class Language(models.Model):
    """
    Model representing a Language (e.g. Russian, English etc.)
    """
    name = models.CharField(max_length=200, 
        help_text="Enter a book language (e.g. Russian, English etc.)")


    def __str__(self):
        """
        String for representing the Model object (in Admin site etc.)
        """
        return self.name


class Book(models.Model):
    """
    Model representing a book (but not a specific copy of a book).
    """
    title = models.CharField(max_length=200)
    author = models.ForeignKey('Author', on_delete=models.SET_NULL, null=True)
    # Foreign Key used because book can only have one author, but authors can have multiple books
    # Author as a string rather than object because it hasn't been declared yet in the file.
    summary = models.TextField(max_length=1000, help_text="Enter a brief description of the book")
    isbn = models.CharField('ISBN',max_length=13, 
        help_text='13 Character <a href="https://www.isbn-international.org/content/what-isbn">ISBN number</a>')
    genre = models.ManyToManyField(Genre, help_text="Select a genre for this book")
    # ManyToManyField used because genre can contain many books. Books can cover many genres.
    # Genre class has already been defined so we can specify the object above.
    language = models.ForeignKey('Language', on_delete=models.SET_NULL, null=True)


    def __str__(self):
        """
        String for representing the Model object.
        """
        return self.title


    def get_absolute_url(self):
        """
        Returns the url to access a particular book instance.
        """
        return reverse('book-detail', args=[str(self.id)])


class BookInstance(models.Model):
    """
    Model representing a specific copy of a book (i.e. that can be borrowed from the library).
    """
    id = models.UUIDField(primary_key=True, default=uuid.uuid4, help_text="Unique ID for this particular book across whole library")
    book = models.ForeignKey('Book', on_delete=models.SET_NULL, null=True)
    imprint = models.CharField(max_length=200)
    due_back = models.DateField(null=True, blank=True)

    LOAN_STATUS = (
        ('m', 'Maintenance'),
        ('o', 'On loan'),
        ('a', 'Available'),
        ('r', 'Reserved'),
    )

    status = models.CharField(max_length=1, choices=LOAN_STATUS, blank=True, default='m', help_text='Book availability')


    class Meta:
        ordering = ["due_back"]


    def __str__(self):
        """
        String for representing the Model object
        """
        return '{0} ({1})'.format(self.id, self.book.title)


class Author(models.Model):
    """
    Model representing an author.
    """
    first_name = models.CharField(max_length=100)
    last_name = models.CharField(max_length=100)
    date_of_birth = models.DateField(null=True, blank=True)
    date_of_death = models.DateField('Died', null=True, blank=True)

    def get_absolute_url(self):
        """
        Returns the url to access a particular author instance.
        """
        return reverse('author-detail', args=[str(self.id)])


    def __str__(self):
        """
        String for representing the Model object.
        """
        return '{0} ({1})'.format(self.last_name, self.first_name)
