Make Reference JSON serializable (#2564)

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# Rationale for this change

## Are these changes tested?

## Are there any user-facing changes?

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Author: Fokko

pyiceberg/expressions/__init__.py

@@ -32,6 +32,8 @@
     Union,
 )
 
+from pydantic import Field
+
 from pyiceberg.expressions.literals import (
     AboveMax,
     BelowMin,
@@ -202,7 +204,7 @@ class UnboundTerm(Term[Any], Unbound[BoundTerm[L]], ABC):
     def bind(self, schema: Schema, case_sensitive: bool = True) -> BoundTerm[L]: ...
 
 
-class Reference(UnboundTerm[Any]):
+class Reference(UnboundTerm[Any], IcebergRootModel[str]):
     """A reference not yet bound to a field in a schema.
 
     Args:
@@ -212,18 +214,18 @@ class Reference(UnboundTerm[Any]):
         An unbound reference is sometimes referred to as a "named" reference.
     """
 
-    name: str
+    root: str = Field()
 
     def __init__(self, name: str) -> None:
-        self.name = name
+        super().__init__(name)
 
     def __repr__(self) -> str:
         """Return the string representation of the Reference class."""
-        return f"Reference(name={repr(self.name)})"
+        return f"Reference(name={repr(self.root)})"
 
-    def __eq__(self, other: Any) -> bool:
-        """Return the equality of two instances of the Reference class."""
-        return self.name == other.name if isinstance(other, Reference) else False
+    def __str__(self) -> str:
+        """Return the string representation of the Reference class."""
+        return f"Reference(name={repr(self.root)})"
 
     def bind(self, schema: Schema, case_sensitive: bool = True) -> BoundReference[L]:
         """Bind the reference to an Iceberg schema.
@@ -242,6 +244,10 @@ def bind(self, schema: Schema, case_sensitive: bool = True) -> BoundReference[L]
         accessor = schema.accessor_for_field(field.field_id)
         return self.as_bound(field=field, accessor=accessor)  # type: ignore
 
+    @property
+    def name(self) -> str:
+        return self.root
+
     @property
     def as_bound(self) -> Type[BoundReference[L]]:
         return BoundReference[L]

tests/expressions/test_expressions.py

@@ -691,6 +691,8 @@ def test_reference() -> None:
     assert repr(ref) == "Reference(name='abc')"
     assert ref == eval(repr(ref))
     assert ref == pickle.loads(pickle.dumps(ref))
+    assert ref.model_dump_json() == '"abc"'
+    assert Reference.model_validate_json('"abc"') == ref
 
 
 def test_and() -> None: