save() silently disregards unsaved Many & ForeignKey relationships

[jayvdb]

If we remove the tag.save(..) at https://github.com/Electron100/butane/blob/master/butane/tests/common/blog.rs#L81 , the many.rs tests fail, but only fail at the assertions on the number of tags after the posts are re-loaded from the database.

Likewise removing the blog.save(..) at https://github.com/Electron100/butane/blob/35ef9d0/butane/tests/common/blog.rs#L91 , the query.rs test fkey_match fails, but it fails trying to fetch the blog, not during the storing of the post in the db.

The Post.save(..) should detect that the related tags are not saved, and either save them, or error. IMO doing the save is the most desirable, and isnt a breaking change as anyone who had code that triggers the related save's had broken code.

[jayvdb]

Due to https://github.com/Electron100/butane/pull/158 , postgres now catches this, resulting in

called `Result::unwrap()` on an `Err` value: Postgres(Error { kind: Db, cause: Some(DbError { severity: "ERROR", parsed_severity: Some(Error), code: SqlState(E23503), message: "insert or update on table \"bar\" violates foreign key constraint \"bar_foo_fkey\"", detail: Some("Key (foo)=(1) is not present in table \"foo\"."), hint: None, position: None, where_: None, schema: Some("public"), table: Some("bar"), column: None, datatype: None, constraint: Some("bar_foo_fkey"), file: Some("ri_triggers.c"), line: Some(2608), routine: Some("ri_ReportViolation") }) })

sqlite ignores it.