Update reflectx to allow for optional nested structs

[ntbosscher]

Nested structs are now only instantiated when one of the database columns in that nested struct is not nil. This allows objects scanned in left/outer joins to keep their natural types (instead of setting everything to NullableX).

Example:

select house.id, owner.*,
from house
left join owner on owner.id = house.owner

Before

type House struct {
 ID      int
 Owner   *Person // if left join gives nulls, Owner is set
}

type Owner struct {
 ID sql.NullInt64 // make nullable columns even tho if table doesn't have those columns nullable
}

After

type House struct {
 ID      int
 Owner   *Person // if left join gives nulls, Owner will be nil
}

type Owner struct {
 ID int    // no need to set this to sql.NullInt
}

[ntbosscher]

A possible solution for #162

[ntbosscher]

One trade off is that for nested structs, we end up doing most of the work in sql.convertAssignRows twice

This also further complicates reflectx.

However, I think the functionality is more natural. With our modeling we typically do things like the following.

type User struct {
    ID int
    Name string
}

type Plan struct {
    ID int
    CreditsPerMonth int64
}

type UserPlan struct {
   User User
   Plan *Plan
}

This maps directly to database tables. Doing things like setting Plan's properties to nullable fields feels weird and out of place.

[ntbosscher]

Happy to make any adjustments if you see a better way to do this.

[Omar-V2]

This makes a lot of sense, I ran into issues with the existing behaviour recently. I don't like that I have to make a bunch of fields Nullable on existing structs just so they can be properly scanned when performing a left join. Any updates from the team if this can get merged? @jmoiron? Thanks!

[maranqz]

@jmoiron, Could you check and merge it? Thanks. 🙏

[phelian]

Another vote for this

[geeeeeeeeek]

Tested locally and it works with multi outer joins. Vote to merge this PR.

[geeeeeeeeek]

Wait it doesn't work with pointer types: don't know how to parse type int64 -> **uint64

type AccountRow struct {
	Account     `db:"account"`
	OptionOrder `db:"option_order"`
}
type OptionOrder struct {
	ID        uuid.UUID `db:"id"`
	AccountID *uint64   `db:"account_id"`
}

[dlsniper]

Hello, @ardan-bkennedy, and I recently stepped in to help maintain this project.
We are sorting the opened issues and pull requests and would like to know if you still NEED this merged.
I think that #900 might solve some issues with this PR and so I'm tempted to close this in favor of that PR.
Thank you for your contribution.