Document is_json as legacy and drop it from assertions

Mark the is_json=true emission in json_field_metadata() as a legacy,
non-standard key kept only for back-compat, to be removed in a future
release. Drop the corresponding assertion from the test helper so the
remaining checks only enforce the canonical Arrow extension keys
(ARROW:extension:name, ARROW:extension:metadata) — the legacy key
continues to be emitted but is no longer part of the contract under
test.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: adriangb

src/common_union.rs

@@ -14,16 +14,22 @@ use datafusion::common::ScalarValue;
 /// Attach this to any Arrow `Field` whose values are JSON-encoded strings so
 /// downstream consumers can recognize them as JSON rather than opaque text.
 ///
-/// Emits both the legacy `is_json` key (for back-compat with existing
-/// consumers of this crate) and Arrow's canonical JSON extension type keys
+/// Emits Arrow's canonical JSON extension type keys
 /// (`ARROW:extension:name` = `arrow.json`, `ARROW:extension:metadata` = `{}`),
 /// see <https://arrow.apache.org/docs/format/CanonicalExtensions.html#json>.
+///
+/// Also emits a legacy `is_json` = `true` key. This key predates this crate's
+/// adoption of the canonical extension and is non-standard — no other Arrow
+/// tool recognizes it. It is kept only for back-compat with existing
+/// downstream consumers of this crate and will be removed in a future
+/// release; new consumers should key off `ARROW:extension:name` instead.
 #[must_use]
 pub fn json_field_metadata() -> HashMap<String, String> {
     HashMap::from([
-        ("is_json".to_string(), "true".to_string()),
         ("ARROW:extension:name".to_string(), "arrow.json".to_string()),
         ("ARROW:extension:metadata".to_string(), "{}".to_string()),
+        // Legacy, non-standard. Remove in a future release — see doc comment above.
+        ("is_json".to_string(), "true".to_string()),
     ])
 }
 

tests/main.rs

@@ -185,7 +185,6 @@ async fn test_json_get_array_inner_field_json_metadata() {
 }
 
 fn assert_json_field_metadata(metadata: &HashMap<String, String>) {
-    assert_eq!(metadata.get("is_json").map(String::as_str), Some("true"));
     assert_eq!(
         metadata.get("ARROW:extension:name").map(String::as_str),
         Some("arrow.json")