Cast datetime columns in sqlite before comparing

[juliusknorr]

When debugging failing integration tests in nextcloud/deck#2934 i noticed that sqlite does strange things when trying to compare a datetime field with a DateTime value through the expression builder. It turns out sqlite doesn't store them as real dates and therefore operates a string comparison through our database abstraction:

https://www.sqlite.org/datatype3.html

2.2. Date and Time Datatype

SQLite does not have a storage class set aside for storing dates and/or times. Instead, the built-in Date And Time Functions of SQLite are capable of storing dates and times as TEXT, REAL, or INTEGER values:

TEXT as ISO8601 strings ("YYYY-MM-DD HH:MM:SS.SSS").
REAL as Julian day numbers, the number of days since noon in Greenwich on November 24, 4714 B.C. according to the proleptic Gregorian calendar.
INTEGER as Unix Time, the number of seconds since 1970-01-01 00:00:00 UTC. 

Applications can chose to store dates and times in any of these formats and freely convert between formats using the built-in date and time functions.

In order to resolve this issue globally this PR wraps the required parameters when they are passed to the query builder with using the DATETIME() function so both values are compared as real datetimes.

Might make sense to also look into covering this with some tests, but I didn't have time to look into yet. I can have a look if the general approach to this is fine.

[nickvergessen]

Looking at https://github.com/nextcloud/server/blob/master/lib/private/DB/QueryBuilder/ExpressionBuilder/OCIExpressionBuilder.php thats a lot of duplication. Xan we abstract the prepareColumn calls to another level?

[juliusknorr]

@nickvergessen I moved the prepareColum to the regular ExpressionBuilder so that sqlite and OCI now just extend that.

[nickvergessen]

Can we add a test? :)

[juliusknorr]

Definitely. I'd probably add a case for this in https://github.com/nextcloud/server/blob/6312c0df6949955d1cd59c3dd444268e0773293c/tests/lib/DB/QueryBuilder/ExpressionBuilderDBTest.php but we'd need a new table in the testing app to properly test this against real databases. Or would you have any other idea to cover that?

[juliusknorr]

Rebased and pushed a test case for this. I added a new table to the testing app to have a datetime column around during tests.