add to and to_column(s) to ColumnLevelConstraint and ModelLevelConstr…

…aint contracts

dbt_common/contracts/constraints.py

@@ -2,7 +2,7 @@
 from enum import Enum
 from typing import Optional, List
 
-from dbt_common.dataclass_schema import dbtClassMixin
+from dbt_common.dataclass_schema import dbtClassMixin, ValidationError
 
 
 class ConstraintType(str, Enum):
@@ -36,8 +36,30 @@ class ColumnLevelConstraint(dbtClassMixin):
     warn_unsupported: bool = (
         True  # Warn if constraint is not supported by the platform and won't be in DDL
     )
+    to: Optional[str] = None
+    to_column: Optional[str] = None
+
+    @classmethod
+    def validate(cls, data):
+        super().validate(data)
+        if data.get("type") is not ConstraintType.foreign_key:
+            if data.get("to") is not None:
+                raise ValidationError(f"Only column-level constraint of type {ConstraintType.foreign_key} can specify a 'to' field.")
+            if data.get("to_column") is not None:
+                raise ValidationError(f"Only column-level constraint of type {ConstraintType.foreign_key} can specify a 'to_column' field.")
 
 
 @dataclass
 class ModelLevelConstraint(ColumnLevelConstraint):
     columns: List[str] = field(default_factory=list)
+    to: Optional[str] = None
+    to_columns: List[str] = field(default_factory=list)
+
+    @classmethod
+    def validate(cls, data):
+        super().validate(data)
+        if data.get("type") is not ConstraintType.foreign_key:
+            if data.get("to") is not None:
+                raise ValidationError(f"Only model-level constraint of type {ConstraintType.foreign_key} can specify a 'to' field.")
+            if data.get("to_columns") is not None:
+                raise ValidationError(f"Only model-level constraint of type {ConstraintType.foreign_key} can specify a 'to_columns' field.")