## Changes to data types loading a pandas DataFrame
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In the absence of schema information, pandas columns with naive
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`datetime64[ns]` values, i.e. without timezone information, are recognized and
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loaded using the `DATETIME` type. On the other hand, for columns with
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timezone-aware `datetime64[ns, UTC]` values, the `TIMESTAMP` type is continued
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to be used.
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## Changes to `Model`, `Client.get_model`, `Client.update_model`, and `Client.list_models`
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The types of several `Model` properties have been changed.
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-`Model.feature_columns` now returns a sequence of `google.cloud.bigquery.standard_sql.StandardSqlField`.
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-`Model.label_columns` now returns a sequence of `google.cloud.bigquery.standard_sql.StandardSqlField`.
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-`Model.model_type` now returns a string.
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-`Model.training_runs` now returns a sequence of dictionaries, as recieved from the [BigQuery REST API](https://cloud.google.com/bigquery/docs/reference/rest/v2/models#Model.FIELDS.training_runs).
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<aname="legacy-protobuf-types"></a>
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## Legacy Protocol Buffers Types
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For compatibility reasons, the legacy proto-based types still exists as static code
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and can be imported:
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```py
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from google.cloud.bigquery_v2 import Model # a sublcass of proto.Message
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```
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Mind, however, that importing them will issue a warning, because aside from
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being importable, these types **are not maintained anymore**. They may differ
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both from the types in `google.cloud.bigquery`, and from the types supported on
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the backend.
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### Maintaining compatibility with `google-cloud-bigquery` version 2.0
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If you maintain a library or system that needs to support both
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`google-cloud-bigquery` version 2.x and 3.x, it is recommended that you detect
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when version 2.x is in use and convert properties that use the legacy protocol
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buffer types, such as `Model.training_runs`, into the types used in 3.x.
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