Remove DataField accessor

[elephantum]

No description provided.

[Copilot]

Pull request overview

Removes the DataField key accessor and standardizes key mapping to only reference meta-table (primary key) columns, simplifying transform key handling across the meta/compute layers.

Changes:

• Removed DataField / FieldAccessor types and updated InputSpec.keys / ComputeInput.keys to dict[str, str].
• Simplified SQL meta aggregation logic to only use meta-table primary keys (no data-table joins for key extraction).
• Updated tests and the key-mapping design doc to reflect the new “keys must be PK/meta columns” model.

Reviewed changes

Copilot reviewed 7 out of 7 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
tests/test_meta_transform_keys.py	Updates schema + key mapping usage to no longer rely on DataField.
design-docs/2025-12-key-mapping.md	Updates design doc examples/constraints to remove DataField and require PK-based keys.
datapipe/types.py	Removes DataField and narrows InputSpec.keys typing to dict[str, str].
datapipe/meta/sql_meta.py	Removes DataField handling and data-table join logic from get_agg_cte.
datapipe/meta/base.py	Updates transform_idx_to_table_idx typing and mapping semantics to dict[str, str].
datapipe/compute.py	Updates ComputeInput.keys typing and removes DataField-based schema logic.
CHANGELOG.md	Removes mention of adding DataField (needs an explicit “removed/breaking change” note).

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