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/02f99246020f_grading_policy.py

@@ -16,12 +16,46 @@ depends_on = None
 
 
 def upgrade():
-    # Add grading policy column to Problem
-    op.add_column('problem', sa.Column('value',
-                                       sa.Enum('set_nothing', 'set_blank', 'set_blank_single', name='gradingpolicy'),
-                                       default=1, nullable=True))
+    # Create copy of problem table
+    op.create_table(
+        'problem_copy',
+        sa.Column('id', sa.Integer(), autoincrement=True, nullable=False),
+        sa.Column('name', sa.Text(), nullable=False),
+        sa.Column('exam_id', sa.Integer(), nullable=False),
+        sa.Column('grading_policy', sa.Enum('set_nothing', 'set_blank', 'set_blank_single', name='gradingpolicy'),
+                  default=1, nullable=False),
+        sa.ForeignKeyConstraint(['exam_id'], ['exam.id'], ),
+        sa.PrimaryKeyConstraint('id')
+    )
+
+    conn = op.get_bind()
+
+    old_problems = conn.execute('SELECT id, name, exam_id FROM problem').fetchall()
+
+    grading_policy = "'set_blank'"
+    for id, name, exam_id in old_problems:
+        # Add quotes for SQLite
+        name = "'" + name + "'"
+        conn.execute(f'INSERT INTO problem_copy VALUES ({id}, {name}, {exam_id}, {grading_policy})')
+
+    op.drop_table('problem')
+    op.rename_table('problem_copy', 'problem')
 
 
 def downgrade():
-    # Remove grading policy column from Problem
-    op.drop_column('problem', 'value')
+    # Create copy of old problem table (without grading policy)
+    op.create_table(
+        'problem_copy',
+        sa.Column('id', sa.Integer(), autoincrement=True, nullable=False),
+        sa.Column('name', sa.Text(), nullable=False),
+        sa.Column('exam_id', sa.Integer(), nullable=False),
+        sa.ForeignKeyConstraint(['exam_id'], ['exam.id'], ),
+        sa.PrimaryKeyConstraint('id')
+    )
+
+    # Move data from new problem table into old problem table
+    op.execute('INSERT INTO problem_copy (id, name, exam_id)' +
+               'SELECT id, name, exam_id FROM problem')
+
+    op.drop_table('problem')
+    op.rename_table('problem_copy', 'problem')