ID variables produce fractional values when mapped between non-hierarchical entities

Problem Description

When mapping ID variables (like household_id, spm_unit_id) between non-hierarchical entities using calculate(..., map_to=...), PolicyEngine Core inappropriately averages these values, producing nonsensical fractional IDs.

Minimal Reproducible Example

from policyengine_us import Microsimulation
import pandas as pd

sim = Microsimulation(dataset='hf://policyengine/policyengine-us-data/enhanced_cps_2024.h5')

# Map household_id to tax_unit level
household_ids_tax_unit = sim.calculate('household_id', map_to='tax_unit')

# Check for fractional values (IDs should always be integers)
fractional_ids = household_ids_tax_unit.values[household_ids_tax_unit.values % 1 != 0]
print(f"Found {len(fractional_ids)} fractional household IDs")
print(f"Examples: {fractional_ids[:5]}")
# Output: [218.  153.5 153.5 172.5 172.5]

Root Cause

In policyengine_core/simulations/simulation.py, the map_result method handles mapping between non-hierarchical group entities (e.g., household → tax_unit) by:

1. First mapping from source to person using how="mean" (averaging)
2. Then mapping from person to target using how="sum" (summing)

This is mathematically inappropriate for ID fields, which are categorical identifiers, not numeric quantities that should be averaged or summed.

Impact

• Produces invalid ID values that break referential integrity
• Can cause silent bugs in downstream analysis
• Affects any code that relies on ID mapping between non-hierarchical entities

Proposed Solutions

1. Short-term: Add a warning when mapping variables with "_id" suffix between non-hierarchical entities
2. Medium-term: Add a variable attribute to mark categorical/ID variables that should not be aggregated mathematically
3. Long-term: Implement proper ID mapping logic that preserves the most common ID or uses a different strategy appropriate for categorical data

Affected Variables

Testing shows at least these ID variables produce fractional values when mapped to tax_unit:

• household_id
• spm_unit_id

Environment

• policyengine-core version: 3.20.0
• policyengine-us version: 1.399.1
• Python version: 3.13

Reproduce the fractional IDs with the minimal example, then inspect policyengine_core/simulations/simulation.py, especially map_result and its person-to-entity mappings. Compare the current mean and sum behavior for household_id and spm_unit_id, and clarify which proposed ID-handling strategy is expected. Done means mapped ID variables no longer produce invalid fractional identifiers and the behavior is covered by an appropriate test.