"""Add unique constraint to ISBN field

Revision ID: 5584e4fd820e
Revises: 0c155d83c55b
Create Date: 2026-03-31 13:41:16.356631

"""
from alembic import op
import sqlalchemy as sa


# revision identifiers, used by Alembic.
revision = '5584e4fd820e'
down_revision = '0c155d83c55b'
branch_labels = None
depends_on = None


def upgrade():
    # ### commands auto generated by Alembic - please adjust! ###
    
    # First, convert empty string ISBNs to a temporary placeholder
    # This is needed because we can't set NULL while column is NOT NULL
    op.execute("UPDATE book SET isbn = '__EMPTY_ISBN__' WHERE isbn = ''")
    
    # Remove duplicate ISBNs, keeping only the book with the lowest ID
    op.execute("""
        DELETE FROM book 
        WHERE isbn != '__EMPTY_ISBN__' 
        AND isbn != '' 
        AND id NOT IN (
            SELECT MIN(id) 
            FROM book 
            WHERE isbn != '__EMPTY_ISBN__' 
            AND isbn != '' 
            GROUP BY isbn
        )
    """)
    
    with op.batch_alter_table('book', schema=None) as batch_op:
        # Make the column nullable
        batch_op.alter_column('isbn',
               existing_type=sa.VARCHAR(length=20),
               nullable=True)
    
    # Now convert the placeholder values to NULL
    op.execute("UPDATE book SET isbn = NULL WHERE isbn = '__EMPTY_ISBN__'")
    
    with op.batch_alter_table('book', schema=None) as batch_op:
        # Add the unique constraint
        batch_op.create_unique_constraint('uq_book_isbn', ['isbn'])

    # ### end Alembic commands ###


def downgrade():
    # ### commands auto generated by Alembic - please adjust! ###
    with op.batch_alter_table('book', schema=None) as batch_op:
        batch_op.drop_constraint('uq_book_isbn', type_='unique')
        batch_op.alter_column('isbn',
               existing_type=sa.VARCHAR(length=20),
               nullable=False)

    # ### end Alembic commands ###