Whenver sources are present, there should be exactly one default source.
Prior to this commit, it was possible to add the first source as a
non-default source and circumvent this invariant. This unfortunately
requires a bit of special-casing logic in s.Add and requires a query to
fetch the number of sources currently persisted.
Tests have been added to cover this as well as deleting the final
source, to ensure that there aren't any odd indexing panics that might
result from trying to find another candidate default source when
deleting.
Previously, when adding a new default source, it was possible to create
more than one default source, since the previous default would not be unset.
Implementing this revealed another issue where deleting the default
source would leave no default set. If the default source is deleted now,
another source is chosen to be the new default.
This ensures that there will only be on default source upon setting a
new default by unsetting the default flag on all other sources. This
only happens when the source to be updated has Default set to true to
avoid a performance hit when updating other attributes.
Chris Goller
Tim Raymond