Word64 does not roundtrip through a database correctly

Bug Reports

The Word64 instance for PersistField does not roundtrip through databases correctly.
It uses fromIntegral for converting between Word64 and Int64.

While this does technically roundtrip, the database values will be silently wrong because negative values will be saved instead of positive ones.

Example:

This does technically roundtrip:

ghci> import Data.Word
ghci> import Data.Int
ghci> 2^64
18446744073709551616
ghci> fromIntegral (18446744073709551615  :: Word64) :: Int64
-1
ghci> fromIntegral (-1 :: Int64) :: Word64
18446744073709551615

BUT the database will contain -1, which does not sort in the same way.

Solutions

You could have another PersistValue constructor for Word64, but I'm not sure if all the "supported" databases support such a type.
As far as I can tell, sqlite supports larger integers by storing them as reals but I don't know about other databases:

sqlite> create table t1 (i INTEGER);
sqlite> insert into t1 values(18446744073709551615);
sqlite> insert into t1 values(0);
sqlite> insert into t1 values(1);
sqlite> select * from t1 order by i;
0
1
1.84467440737096e+19
sqlite> select typeof(i) from t1;
real
integer
integer

Start by locating the Haskell Word64 PersistField instance and the PersistValue handling used by the supported database backends. Compare how each backend stores and orders values beyond Int64, using the SQLite behavior described in the issue as a reference. Done means Word64 values retain their correct values and sort order across supported databases.