I haven't found a way to make the Globals class use the extended definition of the table

You can change the type of a field in a subclass if you declare the field again using the desired type.

Also, simply adding new fields to the subclass does not seem to work

It looks like you are setting instance attributes in the __init__() method, but fields are declared as class attributes.

This example shows a way to add a calculated field records_by_id to ExtendedTable and use ExtendedTable in ExtendedGlobals:

# File: extensions.py
import json
from typing import Any, Dict, List, Optional

from pydantic import Field, validator

from datamodel import Globals, Record, Table


class ExtendedTable(Table):
    # New fields are declared as class attributes not as instance attributes inside the __init__()
    # Calculated fields usually have a default value or default factory so that you don't have to provide a value
    # I prefer a default_factory for mutable values
    records_by_id: Dict[int, Record] = Field(default_factory=dict)

    # A validator can populate a calculated field
    # Use always=True to run the validator even if a value is not supplied and the default value is used
    @validator("records_by_id", always=True)
    def _calculate_records_by_id(
        cls, value: Dict[int, Record], values: Dict[str, Any]
    ) -> Dict[int, Record]:
        records: Optional[List[Record]] = values.get("records")
        if records is None:
            # The records field was not valid
            # Return value or raise a ValueError instead if you want
            return value
        return {record.id: record for record in records}


class ExtendedGlobals(Globals):
    # You can change the type of a field in a subclass if you declare the field again
    table: ExtendedTable


if __name__ == "__main__":
    records = """
    {
        "table": {
            "records": [{"id": 0, "name": "A"}, {"id": 1, "name": "B"}]
        }
    }
    """

    content = json.loads(records)
    extended_globals = ExtendedGlobals.parse_obj(content)
    print(repr(extended_globals))

Output:

ExtendedGlobals(table=ExtendedTable(records=[Record(id=0, name='A'), Record(id=1, name='B')], records_by_id={0: Record(id=0, name='A'), 1: Record(id=1, name='B')}))