Toggling pregrading and Identifying blank solutions

This merge request does two things:

• Automatically identify blank solutions (either multiple choice or not) and set a feedback option named 'blank' for each one.
• When creating an exam, there are three options to specify which questions should be graded automatically.
• Pre-grade a question again if a submission is reuploaded and is automatically graded (by 'Zesje')

The grading policy is specified in the following way:

• 0/set_nothing: Manually approve everything
• 1/set_blank: Automatically approve blank solutions
• 2/set_blank_single: Automatically approve blank solutions or solutions with one answer

Add a new field to a problem in the database with this number.

For a more detailed description, see #61.

Closes #33 and #61.

migrations/versions/d33733e11085_grader_name_unique.py

@@ -34,10 +34,11 @@ def upgrade():
     conn = op.get_bind()
 
     graders = conn.execute('SELECT id, name FROM grader').fetchall()
+    removed_graders = []
 
     for grader in graders:
         # Check if grader is not deleted already
-        if not conn.execute(f'SELECT * FROM grader WHERE grader.id  = {grader.id}').fetchall():
+        if grader in removed_graders:
             continue
 
         # Get other graders with same name
@@ -47,35 +48,38 @@ def upgrade():
             conn.execute(f'UPDATE solution SET grader_id = {grader.id} WHERE solution.grader_id = {other_grader.id}')
             conn.execute(f'DELETE FROM grader WHERE grader.id = {other_grader.id}')
 
+            removed_graders.append(other_grader)
+
+    op.rename_table('grader', 'grader_old')
+
     # Create copy table and remove data
     op.create_table(
-        'grader_copy',
+        'grader',
         sa.Column('id', sa.Integer(), nullable=False),
         sa.Column('name', sa.Text(), nullable=False, unique=True),
         sa.PrimaryKeyConstraint('id')
     )
 
-    op.execute('INSERT INTO grader_copy (id, name)' +
-               'SELECT id, name FROM grader')
+    op.execute('INSERT INTO grader (id, name)' +
+               'SELECT id, name FROM grader_old')
 
-    op.drop_table('grader')
-    op.rename_table('grader_copy', 'grader')
+    op.drop_table('grader_old')
 
 
 def downgrade():
     backup_db()
 
+    op.rename_table('grader', 'grader_old')
+
     # Add Grader table without unique constraint
     op.create_table(
-        'grader_copy',
+        'grader',
         sa.Column('id', sa.Integer(), nullable=False),
         sa.Column('name', sa.Text(), nullable=False, unique=False),
         sa.PrimaryKeyConstraint('id')
     )
 
-    # Move data from old Grader table
-    op.execute('INSERT INTO grader_copy (id, name)' +
-               'SELECT id, name FROM grader')
-
-    op.drop_table('grader')
-    op.rename_table('grader_copy', 'grader')
+    # Move data from old Grader table and delete it
+    op.execute('INSERT INTO grader (id, name)' +
+               'SELECT id, name FROM grader_old')
+    op.drop_table('grader_old')