Merging datasets safely with join keys and validation

                  import pandas as pd

customers = pd.read_parquet('customers.parquet')
orders = pd.read_parquet('orders.parquet')

assert customers['customer_id'].is_unique, 'customer table must be unique by customer_id'

enriched = orders.merge(
    customers[['customer_id', 'plan_tier', 'country']],
    on='customer_id',
    how='left',
    validate='many_to_one',
    indicator=True,
)

unmatched = enriched.loc[enriched['_merge'] != 'both', 'customer_id'].unique()
if len(unmatched) > 0:
    raise ValueError(f'missing customer dimension rows for {len(unmatched)} customer_ids')

enriched = enriched.drop(columns=['_merge'])
print(enriched.head())