Add @IdCompanion annotation.

Author: greenrobot-team

objectbox-java-api/src/main/java/io/objectbox/annotation/IdCompanion.java

@@ -0,0 +1,35 @@
+/*
+ * Copyright 2021 ObjectBox Ltd. All rights reserved.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package io.objectbox.annotation;
+
+import java.lang.annotation.ElementType;
+import java.lang.annotation.Retention;
+import java.lang.annotation.RetentionPolicy;
+import java.lang.annotation.Target;
+
+/**
+ * Marks a property as a companion to an @Id property. There can only be one companion property.
+ * <p>
+ * By defining an @Id companion property, the entity type uses a special ID encoding scheme involving this property
+ * in addition to the ID.
+ * <p>
+ * For Time Series IDs, a companion property of type Date or DateNano represents the exact timestamp.
+ */
+@Retention(RetentionPolicy.CLASS)
+@Target(ElementType.FIELD)
+public @interface IdCompanion {
+}