Improve update

[chochihim]

Can Update be improved?

1. Allow single record be updated by columns (unique key maybe) other than primary key
2. Modify UpdateAll to use type-safe struct instead of models.M
3. Utilise RETURNING clause so that Update, UpdateAll etc methods can return the struct/slice to caller

You can refer to #970 for why I want to make this improvement

[stephenafamo]

I have a suggestion about how this can work.

UpdateAll is modified to use a type-safe argument instead of models.M{}

This directly solves point 2, but also solves point 1, because a single record can be updated using an appropriate query:

_, err := models.UserWallets(
    models.UserWalletWhere.UserID.EQ(userID), // use whatever column matches
).UpdateAll(ctx, db, models.M{
    models.UserWalletColumns.Balance: balance,
})

Returning

1. Add an extra argument to UpdateAll for binding.
   If this extra argument is not nil, a RETURNING clause is added and the result is bound to the argument.
2. If only one record is expected, we can get wallets[0] as our single record. Alternatively, we can also add an UpdateOne() method to UserWalletQuery that adds qm.Limit(1), auto-binds to a UserWallet struct and returns the value.

What do you think @aarondl ?

[aarondl]

@chochihim not against this. But what does the struct look like? There's a real difficulty in doing updates in both REST APIs and sql interfaces when it comes to "is this null? and I should update it? Or is this null and I should not?"

It requires 3 states:

• Omitted: Don't update this
• Null: Set this to null
• Value: Set this to value

Fortunately I could merge in a change that's in the null package that would make this possible and then we -could- use a struct.
See: volatiletech/null#11