preserve db-specific options in schema.rb, fixes #148

[jenseng]

basically any valid db-specific thing that you can already specify via
options: "..." will be preserved on the way back out.

for mysql that's:

• ON UPDATE <action>

for postgres that's:

• ON UPDATE <action>
• ON DELETE SET DEFAULT (doesn't map to a :dependent value)
• DEFERRABLE (NOT DEFERRABLE is the default)
• INITIALLY DEFERRED (INITIALLY IMMEDIATE is the default)
• NOT VALID

postgres' MATCH <type> is not supported, but that's already problematic
for :options since it is incompatible with :dependent (due to where it
appears in the statement), so ¯_(ツ)_/¯

[jenseng]

so i tested this manually in mysql and postgres, and it works great, e.g.

add_foreign_key "enrollments", "users", options: <<-SQL
    ON DELETE SET DEFAULT
    ON UPDATE RESTRICT
    DEFERRABLE INITIALLY DEFERRED
    NOT VALID
  SQL

though i'm wondering what you think about adding some robust end-to-end adapter tests? e.g. in hairtrigger's schema dumper specs, i have it create a db, add tables and triggers, dump them, and then make sure the triggers are dumped correctly ... gives a bit more confidence when changing things

[jenseng]

swapped order of ON UPDATE <-> ON DELETE ... works fine either way, but postgres docs indicate ON DELETE should come first

[jenseng]

hmm though one gotcha is old versions of postgres ... should probably key the convalidated part of the select off of postgres_version >= 90100. all the other stuff is fine all the way back to 7.1 (and rails 3.1+ doesn't support any postgres version older than 8.2)

[matthuhiggins]

I'm not concerned with postgres before 9.0.

[matthuhiggins]

Very cool, thanks a lot @jenseng.

End to end adapter tests would be well received. Maybe some kind of test/dummy app would be necessary.