Poor error-handling of invalid default values makes it hard for users to add new records

[seancolsen]

Interestingly, it's possible for a column's default value to be set to a value which is not actually valid for the column. Consider the following examples:

• A default value set for an FK column and the the referenced record subsequently deleted
• A default value of "blah blah" set for a Mathesar URI or Email column
• A text value set on a column with a unique constraint

If these states arise, it will be impossible for users to add new records to the table. The error looks like this:

Reading that error, it's pretty hard to tell why I couldn't add a new Authors record — and how to fix it!

These default value states are possible in PostgreSQL but are pretty weird. While I wouldn't expect to see states like this in a typical production DB, they are all possible states to produce through Mathesar. And the reason I'm reporting this issue at all is that Amanda and I bumped into a state like this during her product onboarding. We were using the internal site and someone else had apparently set the Authors.Website default value to "Website to be determined" — an invalid URI. I didn't realize this until I spent some time troubleshooting it after our call.

One way to approach this problem would be to validate default values when they are set. But I don't think that would actually get us very far towards addressing the problem here. Various forms of validation can be added after the default value is set, subsequently rendering it invalid.

Ideally we'd have some more graceful error handling, alerting the user to the root cause of the error and instructing them on how to fix it.