DATACMNS-1835 - Provide Type-safe Kotlin query extension.

We now provide a KPropertyPath extension leveraging KProperty references to express a property path. Using Kotlin property references (such as Author::name that translates to `book` or Book::author / Author.name translating to `book.author`) is refactoring-safe as the property expressions are part of the Kotlin language. We render KPropertyPath using the same semantics as our PropertyPath so that store-specific modules can leverage either property paths or accept KProperty directly.

Related ticket: DATAMONGO-2138.

Author: mp911de

src/main/kotlin/org/springframework/data/mapping/KPropertyPath.kt

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+/*
+ * Copyright 2018-2020 the original author or authors.
+ *
+ * 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
+ *
+ *      https://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 org.springframework.data.mapping
+
+import kotlin.reflect.KProperty
+import kotlin.reflect.KProperty1
+
+/**
+ * Abstraction of a property path consisting of [KProperty].
+ *
+ * @author Tjeu Kayim
+ * @author Mark Paluch
+ * @author Yoann de Martino
+ * @since 2.5
+ */
+class KPropertyPath<T, U>(
+		internal val parent: KProperty<U?>,
+		internal val child: KProperty1<U, T>
+) : KProperty<T> by child
+
+/**
+ * Recursively construct field name for a nested property.
+ * @author Tjeu Kayim
+ */
+internal fun asString(property: KProperty<*>): String {
+	return when (property) {
+		is KPropertyPath<*, *> ->
+			"${asString(property.parent)}.${property.child.name}"
+		else -> property.name
+	}
+}
+
+/**
+ * Builds [KPropertyPath] from Property References.
+ * Refer to a nested property in an embeddable or association.
+ *
+ * For example, referring to the field "author.name":
+ * ```
+ * Book::author / Author::name isEqualTo "Herman Melville"
+ * ```
+ * @author Tjeu Kayim
+ * @author Yoann de Martino
+ * @since 2.5
+ */
+operator fun <T, U> KProperty<T?>.div(other: KProperty1<T, U>) =
+		KPropertyPath(this, other)

src/main/kotlin/org/springframework/data/mapping/KPropertyPathExtensions.kt

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+/*
+ * Copyright 2020 the original author or authors.
+ *
+ * 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
+ *
+ *      https://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 org.springframework.data.mapping
+
+import kotlin.reflect.KProperty
+
+/**
+ * Extension for [KProperty] providing an `toPath` function to render a [KProperty] as property path.
+ *
+ * @author Mark Paluch
+ * @since 2.5
+ */
+fun KProperty<*>.toPath(): String = asString(this)

src/test/kotlin/org/springframework/data/mapping/KPropertyPathTests.kt

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+/*
+ * Copyright 2018-2020 the original author or authors.
+ *
+ * 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
+ *
+ *      https://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 org.springframework.data.mapping
+
+import org.assertj.core.api.Assertions.assertThat
+import org.junit.Test
+
+/**
+ * Unit tests for [KPropertyPath] and its extensions.
+ *
+ * @author Tjeu Kayim
+ * @author Yoann de Martino
+ * @author Mark Paluch
+ */
+class KPropertyPathTests {
+
+	@Test // DATACMNS-1835
+	fun `Convert normal KProperty to field name`() {
+
+		val property = Book::title.toPath()
+
+		assertThat(property).isEqualTo("title")
+	}
+
+	@Test // DATACMNS-1835
+	fun `Convert nested KProperty to field name`() {
+
+		val property = (Book::author / Author::name).toPath()
+
+		assertThat(property).isEqualTo("author.name")
+	}
+
+	@Test // DATACMNS-1835
+	fun `Convert double nested KProperty to field name`() {
+
+		class Entity(val book: Book)
+
+		val property = (Entity::book / Book::author / Author::name).toPath()
+
+		assertThat(property).isEqualTo("book.author.name")
+	}
+
+	@Test // DATACMNS-1835
+	fun `Convert triple nested KProperty to field name`() {
+
+		class Entity(val book: Book)
+		class AnotherEntity(val entity: Entity)
+
+		val property = asString(AnotherEntity::entity / Entity::book / Book::author / Author::name)
+
+		assertThat(property).isEqualTo("entity.book.author.name")
+	}
+
+	@Test // DATACMNS-1835
+	fun `Convert simple KProperty to property path using toPath`() {
+
+		class AnotherEntity(val entity: String)
+
+		val property = AnotherEntity::entity.toPath()
+
+		assertThat(property).isEqualTo("entity")
+	}
+
+	@Test // DATACMNS-1835
+	fun `Convert nested KProperty to field name using toPath()`() {
+
+		val property = (Book::author / Author::name).toPath()
+
+		assertThat(property).isEqualTo("author.name")
+	}
+
+	@Test // DATACMNS-1835
+	fun `Convert triple nested KProperty to property path using toPath`() {
+
+		class Entity(val book: Book)
+		class AnotherEntity(val entity: Entity)
+
+		val property = (AnotherEntity::entity / Entity::book / Book::author / Author::name).toPath()
+
+		assertThat(property).isEqualTo("entity.book.author.name")
+	}
+
+	@Test // DATACMNS-1835
+	fun `Convert nullable KProperty to field name`() {
+
+		class Cat(val name: String?)
+		class Owner(val cat: Cat?)
+
+		val property = asString(Owner::cat / Cat::name)
+		assertThat(property).isEqualTo("cat.name")
+	}
+
+	class Book(val title: String, val author: Author)
+	class Author(val name: String)
+}