coalesce: use Coalesce class, handle pre_tuples

querybuilder/atoms/columns.py

@@ -142,8 +142,8 @@ class Column(Atom):
 
     @columnize
     @method_accepting_lambdas
-    def coalesce(self, *others):
-        return Transform("COALESCE", self.pretuple_with(*others))
+    def coalesce(self, *others, **kwargs):
+        return Coalesce(self.pretuple_with(*others), **kwargs)
 
     def compress(self, *, unsafe=False):
         """Method to compress a column into an equivalent one
@@ -928,8 +928,8 @@ class Pretuple(Expression, Generic[Col]):
         sqltype = tuple((c.sqltype for c in columns))
         super().__init__(columns=columns, sqltype=sqltype, **kwargs)
 
-    def coalesce(self, **kwargs):
-        return Coalesce(self, **kwargs)
+    def coalesce(self, *others, **kwargs):
+        return self.columns[0].__class__.coalesce(*self.columns, *others, **kwargs)
 
     def to_placeholder(self, key: Optional[str | _MISSING_TYPE] = None) -> Placeholder:
         raise NotImplementedError()
@@ -972,6 +972,7 @@ class Pretuple(Expression, Generic[Col]):
     @columnize
     @method_accepting_lambdas
     def pretuple_with(self, *others):
+        # TODO: Why not done as follows in Column class?
         if not others:
             return self