Add isFile column in url table

[yong-jie]

Problem

We need to introduce a column in the url table to indicate whether a shortlink is pointing to a file.go.gov.sg object.

Solution

Create two scripts. The first would be to add the isFile column in a backwards-compatible way. This is done by making the column nullable(already true by default). After future PRs that update the codebase to start writing into the isFile column, run the second script to backfill and set the not-null constraint.

Follow up

I'll PR the next step which updates the url model after the migration strategy has been finalised and merged.

[liangyuanruo]

Thought through the use of DEFAULT false, still have to

• backfill false values for standard short links
• backfill url_history

[yong-jie]

@liangyuanruo I've changed the approach back to the one you were suggesting, involving two steps. The first being to set a nullable column in both tables, and then the second script to backfill + add not-null constraint.

[liangyuanruo]

As commented

[yong-jie]

target branch has been changed to develop, requesting review before merge so i can get the next steps up

[liangyuanruo]

lgtm!