@GeneratedColumn

[gavinking]

We have a @ColumnDefault annotation which works nicely in conjunction with @Generated(INSERT). But some databases have a syntax like generated as (....) which is a bit more powerful than default.

In #4220 I tried supporting this by adding an as member to @Generated, but perhaps that's not the right place to put it. So what are the other options? A new annotation, @ColumnGeneratedAs or @GeneratedAs?

Suggestions?

[gavinking]

Another closely related issue: I'm starting to think it's silly to have to write @Generated(INSERT) @ColumnDefault and @Generated(ALWAYS) @ColumnGeneratedAlways.

I think we could make:

• @ColumnDefault imply @Generated(INSERT) and
• @ColumnGeneratedAlways imply @Generated(ALWAYS).

I know it's a bit out of character to have the DDL annotations imply stuff at the object level rather than the other way around, but it makes perfect sense in this case.

[gavinking]

After discussion with @sebersole we've identified two remaining issues:

The name

@ColumnGeneratedAlways is bad.

I now lean toward @GeneratedColumn, but alternatives include @GeneratedAlways or @AlwaysGenerated.

Multiple-column mappings

This annotation is currently—like its friend @ColumnDefault—limited to single-column mappings.

There are a range of imperfect solutions to this:

1. Disambiguate the column by name

   @GeneratedColumn(name="total", value="quantity*unitPrice")
   

   I feel like this is a bit fragile if you have stuff like naming strategies, but it's in-principle more powerful since you can use it in conjunction with @JoinColumn, @DiscriminatorColumn, etc.

2. Embed a @Column annotation

   @GeneratedColumn(column=@Column(name="total"), value="quantity*unitPrice")
   

   This is less fragile but also less flexible.

3. Copy/paste all the members of @Column into @GeneratedColumn. Icky, but convenient for the user.

Note that what we decide to do here for @GeneratedColumn also needs to work for @ColumnDefault. On balance I think option 1 is best, as long as the name was optional for single-column mappings.

[gavinking]

After going down a somewhat deeper rabbit hold than anticipated, and discovering that there are reasonable ways to use @ColumnDefault (some documented, some not, and none covered by the tests) that really don't fit into the same usage pattern as @GeneratedColumn, I've decided not to shoehorn together these two annotations.

[sebersole]

At first glance, I think it only makes sense to allow this on basic attributes. On that premise, I think the ability to name the column is unnecessary. Also on that premise, I think the annotation actually should be geared toward the attribute rather than the column. As you say, it describes aspects of both an typically we "push these down" to the db level. Given all of that, I'd lean towards:

@AlwaysGenerated( "quantity*unitPrice" )
@Column( name="total" )

[sebersole]

Though to match better with the phrasing on the SQL side, @GeneratedAlways might be better

[gavinking]

@sebersole I totally missed your comments here.

Do you want me to remove name from these annotations (I can totally do it, it just means deleting code).