Support column DEFAULT when inserting/updating via struct

[aheuermann]

When inserting/updating by struct, I couldn't find a way to force the column DEFAULT.

INSERT into table (column_with_default) values (DEFAULT);
UPDATE table SET column_with_default = DEFAULT;

Some ideas:

1: Add a tag like defaultifempty. This would work well for most use-cases, but wouldn't allow you to set the column to the "zero value" if desired (you would need to use a map).

  type Person struct {
    FirstName string     `goqu:defaultifempty`
    Last Name NullString `goqu:defaultifempty`
  }
  p := Person {
    FirstName: "",
    LastName: NullString {
      String: "",
      Valid: true
    }
  } //both would use database default when inserted/updated

2: Add a Default interface. This would require you to wrap things like NullString. I don't particularly like this idea. NullString is verbose/confusing as-is; adding another boolean for default makes it worse, but it would handle all use-cases.

type Person struct {
  Name NullStringWithDefault
}

p := Person {
  Name: NullStringWithDefault("", false, true)
} //would insert as DEFAULT

p2 := Person {
  Name: NullStringWithDefault("", false, false)
} //would insert as NULL

p3 := Person {
  Name: NullStringWithDefault("", true, true)
} //would this insert as NULL, DEFAULT, or error?

If anything, I think I like _option 1_ since it is simple and it would work for most use-cases. For cases where you want the zero value, you can always use a map. Let me know what you think. Maybe there are better solutions I'm missing.

[chen56]

The override value should be simpler

	dialect.From("person").
		Insert(p).
	        WithOverride("name",func()interface{}{
			if p.Name==""{
				return goqu.L("DEFAULT")
			}else{
				return sex
			}
		}).Exec()

or

	dialect.From("person").
		Insert(p).
		WithOverride("name",goqu.L("DEFAULT"),func()bool{
			var isOverride = p.Name==""
			return isOverride
		}).Exec()

or

	ds:=dialect.From("person").Insert(p) 
        if p.Name!=""{
             ds.WithOverride("name",goqu.L("DEFAULT"))
        }

[doug-martin]

I went with option 1 for now because it was fairly trivial to implement. If we end up needing the WithOverride type functionality later we can consider it.