sqlserver_column.py: Handle string dtype of nvarchar

nvarchar is not considered a string in the base adapter, nor in the fabric adapter. This leads to issues when dealing with nvarchar columns, such as in https://github.com/dbt-msft/dbt-sqlserver-utils/blob/master/macros/sql/union.sql

In this commit we add nvarchar to the list of string dtypes, return the correct string_size for nvarchar (which is doubled by default), and use the correct dtype in string_type

Author: Cogito

dbt/adapters/sqlserver/sqlserver_column.py

@@ -20,3 +20,31 @@ def is_integer(self) -> bool:
             "serial8",
             "int",
         ]
+
+    def is_string(self) -> bool:
+        return self.dtype.lower() in [
+            "text",
+            "character varying",
+            "character",
+            "varchar",
+            "nvarchar",
+        ]
+
+    def string_size(self) -> int:
+        if not self.is_string():
+            raise DbtRuntimeError("Called string_size() on non-string field!")
+
+        if self.dtype == "text" or self.char_size is None:
+            # char_size should never be None. Handle it reasonably just in case
+            return 256
+        elif self.dtype.lower() == "nvarchar":
+            # char_size is doubled for nvarchar
+            return int(self.char_size // 2)
+        else:
+            return int(self.char_size)
+
+    def string_type(self, size: int) -> str:
+        if self.dtype:
+            return f"{self.dtype}({size if size > 0 else '8000'})"
+        else:
+            return f"varchar({size if size > 0 else '8000'})"