Persist structures with no data fields besides id

I have a mapping like this:

```
type Product struct {
Id int
PropertyGroup PropertyGroup `ref:"property_group_id" fk:"id" autosave:"true" autoload:"true"`
PropertyGroupId int
}

type PropertyGroup struct {
Id int
Properties []Property `ref:"id" fk:"group_id" autosave:"true" autoload:"true"`
}

type Property struct {
Id int
Key string
Value string

Group PropertyGroup `ref:"group_id" fk:"id"`
GroupId int
}
```

Please note that what's important here is the PropertyGroup, which has no data fields. I kept Property in sample only to show what I want to achieve and why I might need such an empty table.

When doing the insert for Product, with a reference to an existing PropertyGroup, I have this error:
`pq: syntax error at or near "WHERE"`

Generated sql is this:
`UPDATE "property_groups" SET WHERE "property_groups"."id"=$1; - pq: syntax error at or near "WHERE"`

Of course an workaround might be to remove the autosave:"true" between Product and PropertyGroup, but this would break the insert when the PropertyGroup doesn't exist.