I'm experimenting with reflections on Nim, mostly to get a better grasp of the
language, seeing if I can make a prepared SQL statement given any object with
any number of fields. It works, but there are a few things I still didn't
manage. Here's the code:
import strutils
proc createTable[Obj](): string =
var tostr: Obj
#The SQL should be compatible with PostgreSQL and MySQL/MariaDB
var preparedSql = "CREATE OR REPLACE TABLE "
preparedSql.add($Obj)
preparedSql.add(" (")
var vartype : string
for name, field in fieldPairs(tostr):
preparedSql.add(name)
preparedSql.add(" ")
if ($field.type=="string"):
vartype = "varchar(3000)"
else:
vartype = $(field.type)
preparedSql.add(vartype)
preparedSql.add(", ")
preparedSql.removeSuffix(", ") #This could be an if check in the for loop
above, but this works fine right now.
preparedSql.add(");")
result = preparedSql #Might as well use result instead of preparedSql in
the code above
type Sample = object
pid : int
forkey : int
name : string
description : string
var sam = Sample(pid: 1, forkey: 2, name: "Sample", description: "Plenty of
words")
echo createTable[sam]()
#The result will print: CREATE OR REPLACE TABLE Sample (pid int, forkey
int, name varchar(3000), description varchar(3000));
Run
I do think the code is much longer than needed, but I'm not entirely sure what
could be trimmed/joined.
Now, there are some questions I'd like to make:
1. Why does fieldPairs() only work when called for an iterator? Is there a
way to treat its result as a variable or something that can be printed/echo'ed?
2. Why does the generic "accept any type of object" have to be declared in
the brackets instead of parenthesis of a proc? I tried `proc createTable (obj:
T)` and `(obj: object)`, none worked.
3. I am currently looking into custom annotations, which are done with
pragmas defined as templates. Will the reflection recognize them? How do I
check for their existence to properly treat each object field?
4. Other than annotations/pragmas, is it possible to give extra details to
object's fields, such as defining a primary key, foreign keys, if it is
nullable, etc?
5. What would be a decent alternative to the several `preparedSql.add()` that
I call one after another?
