fix: Validate SetStatisticsUpdate correctly (fixes #2865)

[ragnard]

Previously the pydantic @model_validator on SetStatisticsUpdate would fail because it assumed statistics was a model instance. In a "before"" validator that is not the case.

Use an "after" validator instead, where we can use instantiated and validated fields.

Before

>>> import pyiceberg.table.update
>>> pyiceberg.table.update.SetStatisticsUpdate.model_validate({'statistics': {'snapshot-id': 1234, 'file-size-in-bytes': 0, 'statistics-path': '', 'file-footer-size-in-bytes': 0, 'blob-metadata': []}})
Traceback (most recent call last):
  File "<python-input-1>", line 1, in <module>
    pyiceberg.table.update.SetStatisticsUpdate.model_validate({'statistics': {'snapshot-id': 1234, 'file-size-in-bytes': 0, 'statistics-path': '', 'file-footer-size-in-bytes': 0, 'blob-metadata': []}})
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/ragge/projects/github.com/ragnard/iceberg-python/.venv/lib/python3.14/site-packages/pydantic/main.py", line 716, in model_validate
    return cls.__pydantic_validator__.validate_python(
           ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^
        obj,
        ^^^^
    ...<5 lines>...
        by_name=by_name,
        ^^^^^^^^^^^^^^^^
    )
    ^
  File "/home/ragge/projects/github.com/ragnard/iceberg-python/pyiceberg/table/update/__init__.py", line 191, in validate_snapshot_id
    data["snapshot_id"] = stats.snapshot_id
                          ^^^^^^^^^^^^^^^^^
AttributeError: 'dict' object has no attribute 'snapshot_id'

After

>>> import pyiceberg.table.update
>>> pyiceberg.table.update.SetStatisticsUpdate.model_validate({'statistics': {'snapshot-id': 1234, 'file-size-in-bytes': 0, 'statistics-path': '', 'file-footer-size-in-bytes': 0, 'blob-metadata': []}})
SetStatisticsUpdate(action='set-statistics', statistics=StatisticsFile(snapshot_id=1234, statistics_path='', file_size_in_bytes=0, file_footer_size_in_bytes=0, key_metadata=None, blob_metadata=[]), snapshot_id=1234)

Rationale for this change

Are these changes tested?

Yes, but only using the two-liners above.

Are there any user-facing changes?

No.

[kevinjqliu]

Thanks for the PR. I think we should just deprecate the top-level snapshot_id entirely.
For context, the "before model_validator` was added in 3b53edc#diff-769b43e1d8beaa86141f694679de2bbea3604a65f987a9acd7d9e9efca193b7eR181-R193 to maintain backwards compatibility and prep for deprecation

[kevinjqliu]

oops didnt mean to approve

[ragnard]

@kevinjqliu Ok, do you want me to change the fix so that snapshot_id is still there, but just not automatically populated?

[kevinjqliu]

Looks like the java implementation has not deprecated the top-level snapshot_id. lets proceed with this change since it improves the current validation logic.

Thanks for the PR! Lets add a test and it should be good to go!

[kevinjqliu]

Suggested change

	@model_validator(mode="after")
	def validate_snapshot_id(self) -> Self:
	return self.model_copy(update={"snapshot_id": self.statistics.snapshot_id})
	@model_validator(mode="after")
	def validate_snapshot_id(self) -> Self:
	self.snapshot_id = self.statistics.snapshot_id
	return self

nit: use direct assignment

[kevinjqliu]

add a test, something like:

def test_set_statistics_update_without_snapshot_id(table_v2_with_statistics: Table) -> None:
    """Test that SetStatisticsUpdate auto-populates snapshot_id from statistics."""
    snapshot_id = table_v2_with_statistics.metadata.current_snapshot_id

    blob_metadata = BlobMetadata(
        type="apache-datasketches-theta-v1",
        snapshot_id=snapshot_id,
        sequence_number=2,
        fields=[1],
        properties={"prop-key": "prop-value"},
    )

    statistics_file = StatisticsFile(
        snapshot_id=snapshot_id,
        statistics_path="s3://bucket/warehouse/stats.puffin",
        file_size_in_bytes=124,
        file_footer_size_in_bytes=27,
        blob_metadata=[blob_metadata],
    )

    # Create update without explicitly providing snapshot_id
    update = SetStatisticsUpdate(statistics=statistics_file)

    # Validator should auto-populate snapshot_id from statistics
    assert update.snapshot_id == snapshot_id

    new_metadata = update_table_metadata(
        table_v2_with_statistics.metadata,
        (update,),
    )

    assert len(new_metadata.statistics) == 2
    updated_statistics = [stat for stat in new_metadata.statistics if stat.snapshot_id == snapshot_id]
    assert len(updated_statistics) == 1