MultiPolygon missing one array level reports invalid geometry

• Console: Error: Invalid MultiPolygon coordinates: coordinates must be an array of Polygon coordinate arrays.
• Leaflet logs nothing and simply renders that feature not at all.
• QGIS reports that the GeoJSON is invalid and the geometry cannot be parsed.

1. Print geometry.coordinates and check whether level 2 holds arrays or numbers.
2. Count nesting depth recursively and compare with the spec: Point 1, LineString 2, Polygon 3, MultiPolygon 4.
3. Check whether the backend hand-assembled the geometry instead of using ST_AsGeoJSON.
4. Open the file in QGIS or run turf.cleanCoords to see the rejection.
5. Confirm the inner content really is polygons and not rings before adding a wrapper.

Use ST_Multi in SQL so PostGIS emits the four-level structure, or wrap the existing three-level coordinates in one more array on the client: coordinates: [geom.coordinates].

function depth(a) {
  return Array.isArray(a) ? 1 + Math.max(...a.map(depth)) : 0;
}
depth(fc.features[0].geometry.coordinates);   // 3 -> 应为 4
console.log(fc.features[0].geometry.coordinates[0]);
// 正确：[ [ [116.1,39.8], [116.2,39.8], ... ] ]

-- 正确：交给 PostGIS 生成 MultiPolygon
SELECT jsonb_build_object(
  'type', 'Feature',
  'properties', jsonb_build_object('name', name),
  'geometry', ST_AsGeoJSON(ST_Multi(geom), 15)::jsonb
)
FROM districts;

-- 若只有一个面，ST_Multi 也会生成合法的 MultiPolygon

• Hand-editing geometry.type from Polygon to MultiPolygon without adding the extra array level.
• Adding two wrapper levels and turning each ring into a degenerate polygon.