Improve error messages for composite keys

[npatki]

Environment Details

• SDV version: 1.7.0 (latest)

Problem

Currently, composite keys are not supported by any of the synthesizers in the SDV. Yet, some parts of the metadata functionality seem to imply that it is allowed. Until we add support for composite keys (see #613), the metadata should only allow for single-column primary keys.

Example 1: Python Dict

The following metadata has composite keys but it seems to pass the validation check.

from sdv.metadata import MultiTableMetadata

metadata_dict = {
    'tables': {
        'parent': {
            'primary_key': ('id', 'id2'),
            'columns': {
                'id': { 'sdtype': 'id'},
                'id2': { 'sdtype': 'id' },
                'col_A': { 'sdtype': 'numerical' },
            }
            
        },
        'child': {
            'columns': {
                'A_id': { 'sdtype': 'id'},
                'A_id2': { 'sdtype': 'id'},
                'col_C': { 'sdtype': 'numerical'}
            }
            
        }
    },
    'relationships': [{
        'parent_table_name': 'parent',
        'child_table_name': 'child',
        'parent_primary_key': ('id', 'id2'),
        'child_foreign_key': ('A_id', 'A_id2')
    }]
}

metadata = MultiTableMetadata.load_from_dict(metadata_dict)
metadata.validate()

Expectation: I expect that this should not pass validation. It should produce an error:

InvalidMetadataError: The metadata is not valid

Table: parent
'primary_key' must be a string.

Example 2: JSON

In JSON, it is not possible to encode a tuple so many users may try to encode a composite key as a list. This creates a confusing error that seems to imply that using tuples would work.
metadata_example.json

from sdv.metadata import MultiTableMetadata

metadata = MultiTableMetadata.load_from_json(
    filepath='metadata_example.json')

metadata.validate()

InvalidMetadataError: The metadata is not valid

Table: parent
'primary_key' must be a string or tuple of strings.

Expectation: The error message should not imply that a tuple of strings is even a possibility. Because tuples are not allowed in JSON and also we do not support composite keys. The error message should only indicate that it can be a string.

InvalidMetadataError: The metadata is not valid

Table: parent
'primary_key' must be a string.