Instant should persist the same way (BsonType.DATE_TIME) as LocalDateTime and Date

UPGRADE7.md

@@ -49,4 +49,4 @@ Experienced while upgrading modules for Grails 7
 ### Cool New Features
 - Hello Exterminator, Good by bugs! Lot's of things started working... and working well! For instance, use of controller namespaces now work seemlessly.
 - Massive decoupling of dependencies and cleanup between modules.
-- Works with Spring Security 6 out of the box. No plugin needed!
+- Works with Spring Security 6 out of the box. No plugin needed!

grails-data-mongodb/bson/src/main/groovy/org/grails/datastore/bson/codecs/temporal/InstantBsonConverter.groovy

@@ -39,17 +39,17 @@ trait InstantBsonConverter implements TemporalBsonConverter<Instant>, InstantCon
 
     @Override
     void write(BsonWriter writer, Instant value) {
-        writer.writeInt64(convert(value))
+        writer.writeDateTime(convert(value))
     }
 
     @Override
     Instant read(BsonReader reader) {
-        convert(reader.readInt64())
+        convert(reader.readDateTime())
     }
 
     @Override
     BsonType bsonType() {
-        BsonType.INT64
+        BsonType.DATE_TIME
     }
 
 }

grails-data-mongodb/bson/src/test/groovy/org/grails/datastore/bson/codecs/temporal/InstantBsonConverterSpec.groovy

@@ -38,7 +38,7 @@ class InstantBsonConverterSpec extends Specification implements InstantBsonConve
     void "test read"() {
         given:
         BsonReader bsonReader = Mock(BsonReader) {
-            1 * readInt64() >> 100L
+            1 * readDateTime() >> 100L
         }
 
         when:
@@ -56,11 +56,11 @@ class InstantBsonConverterSpec extends Specification implements InstantBsonConve
         write(bsonWriter, instant)
 
         then:
-        1 * bsonWriter.writeInt64(100L)
+        1 * bsonWriter.writeDateTime(100L)
     }
 
     void "test bson type"() {
         expect:
-        bsonType() == BsonType.INT64
+        bsonType() == BsonType.DATE_TIME
     }
 }

grails-doc/src/en/guide/upgrading/upgrading60x.adoc

@@ -557,4 +557,16 @@ tasks.withType(Test).configureEach {
 tasks.withType(JavaExec).configureEach {
    jvmArgs('-XX:+TieredCompilation', '-XX:TieredStopAtLevel=1', '-XX:CICompilerCount=3')
 }
+----
+
+===== 12.24 MongoDB
+
+`Instant` persistence switched from epoch milliseconds (BSON int64) to BSON DateTime (same BSON type as `java.util.Date`); `LocalDateTime` continues to be converted to BSON DateTime using the configured zone. If you currently using domain objects with property type `Instant`, you will need to convert them prior to upgrading. For more info, see https://github.com/apache/grails-core/pull/15111[#15111].
+
+[source,javascript]
+----
+db.Example.updateMany(
+  { created: { $type: "long" } },
+  [ { $set: { created: { $toDate: "$created" } } } ]
+);
 ----