Introduce Answer.course_key and deprecate course_id.

[mtyaka]

This patch introduces a new 255-char course_key field to the Answer model. The old 50-char course_id field is still present, but is deprecated and will be removed in next release.

It removes the previous migration (which was never deployed to production) which extended the existing course_id field to 255 characters, which was found to not be the optimal approach on the edx.org production database due to the relevant table's size. We cannot simply extend the existing course_id field because we have some large problem_builder_answer tables in production and the migration to extend the column would lock the table causing issues in production.

The code is updated so that it uses the new course_key column, but falls back to course_id.
In next release, a data migration to copy course_id data to course_key will be provided and the course_id column dropped.

Below is the output of sqlmigrate for the new migration:

BEGIN;
ALTER TABLE `problem_builder_answer` ADD COLUMN `course_key` varchar(255) NULL;
ALTER TABLE `problem_builder_answer` ADD CONSTRAINT `problem_builder_answer_student_id_2ce682a818c95cbc_uniq` UNIQUE (`student_id`, `course_key`, `name`);
CREATE INDEX `problem_builder_answer_c8235886` ON `problem_builder_answer` (`course_key`);

COMMIT;

Next release will add a data migration to copy contents of course_id to the new course_key column. EdX will fake this migration and copy data in batches instead.
Next release will also drop the deprecated course_id column.

See https://github.com/edx/edx-platform/pull/14013#issuecomment-261635454 for more information.

Environments: edx.org and edge.edx.org

Merge deadline: ASAP - this is blocking a course on Edge

JIRA tickets: TNL-5932

Discussions: See https://github.com/edx/edx-platform/pull/14013 and JIRA ticket.

Sandbox URL:

• LMS: http://pr14044.sandbox.opencraft.hosting/
• Studio: http://studio-pr14044.sandbox.opencraft.hosting/

Partner information: hosted on edX Edge (Davidson College)

Testing instructions:

Check that you can successfully add a Problem Builder "Long Answer" component to a course with a course key that is longer than 50 character and that answers answered on an older version of problem-builder continue to work correctly with this patch.

1. Install problem-builder v2.6.1
2. Create a course with a combined course key less than 50 characters.
3. Add a Problem Builder component, then add the Long Answer component.
4. Answered several questions.
5. Create a course with a combined course key more than 50 characters.
6. Add Unit with Problem Builder and a Long Answer element - watched it fail.
7. Installed problem-builder from this PR.
8. Run migrations.
9. Verify that the answers are still present in the first course.
10. Verify that the Long Answer element in the course with the very long course ID is working now, and you can submit answers to it.

Reviewers

• @pomegranited

[pomegranited]

👍 @mtyaka

• I tested this on my devstack:
  1. Installed problem-builder v2.6.1
  2. Created a course, and imported the Problem Builder Demo Course.
  3. Answered several questions.
  4. Created a test course with a very long course ID
  5. Added Unit with Problem Builder and a Long Answer element - watched it fail.
  6. Installed problem-builder from this PR
  7. Ran migrations
  8. Verified that my answers are still present in the Demo course.
  9. Verified that the Long Answer element in the course with the very long course ID is working now, and I can submit answers to it.
• I tested this on the sandbox instance:
  1. Created a course with a course ID > 50 chars
  2. Added Unit with Problem Builder and a Long Answer element.
  3. Verified that I could answer the question in the LMS.
• I read through the code
• I checked for accessibility issues
• Includes documentation

[pomegranited]

Will the uniqueness constraint ('student_id', 'course_key', 'name') get us into trouble if all the initial course_key values are null?

[mtyaka]

No, multiple NULL values are not considered to be identical to each other so they don't violate the uniqueness constraint.

[mtyaka]

Rebased from f0d0ce5.

[mtyaka]

Thank you @pomegranited!

[maxrothman]

Why are you writing raw SQL here rather than using the ORM?

[mtyaka]

@maxrothman Only because I was not familiar with django Q objects and didn't know django ORM supports OR queries. Thanks for the hint, I opened #136 to convert the raw SQL query to Q objects - can you please review?

If it looks good, I'll update the version number at https://github.com/edx/edx-platform/pull/14044 and we can finally get that one merged.