Make ISBN field nullable and add unique constraint

migrations/versions/5584e4fd820e_add_unique_constraint_to_isbn_field.py

@@ -0,0 +1,64 @@
+"""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 ###