#521 - Add example for type-safe Kotlin MongoDB query DSL.

Original pull request: #522.

README.md

@@ -27,7 +27,7 @@ We have separate folders for the samples of individual modules:
 * `geo-json` - Example project showing usage of [GeoJSON](http://geojson.org) with MongoDB.
 * `gridfs` - Example project showing usage of gridFS with MongoDB.
 * `java8` - Example of how to use Spring Data MongoDB with Java 8 date time types as well as the usage of `Optional` as return type for repository methods. Note, this project requires to be build with JDK 8.
-* `kotlin` - Example for using Cassandra with MongoDB.
+* `kotlin` - Example for using [Kotlin](https://kotlinlang.org/) with MongoDB.
 * `query-by-example` - Example project showing usage of Query by Example with MongoDB.
 * `querydsl` - Example project showing imperative and reactive [Querydsl](https://github.com/querydsl/querydsl) support for MongoDB.
 * `reactive` - Example project to show reactive template and repository support.

mongodb/kotlin/README.md

@@ -54,3 +54,12 @@ interface PersonRepository : CrudRepository<Person, String> {
 	fun findOneByFirstname(firstname: String): Person
 }
 ```
+
+## Type-Safe Kotlin Mongo Query DSL
+
+Using the `Criteria` extensions allows to write type-safe queries via an ideomatic API.
+
+```kotlin
+operations.find<Person>(Query(Person::firstname isEqualTo "Tyrion"))
+```
+

mongodb/kotlin/src/test/kotlin/example/springdata/mongodb/people/MongoDslTests.kt

@@ -0,0 +1,70 @@
+/*
+ * Copyright 2019 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 example.springdata.mongodb.people;
+
+import org.assertj.core.api.Assertions.assertThat
+import org.junit.Before
+import org.junit.Test
+import org.junit.runner.RunWith
+import org.springframework.beans.factory.annotation.Autowired
+import org.springframework.boot.test.context.SpringBootTest
+import org.springframework.data.mongodb.core.MongoOperations
+import org.springframework.data.mongodb.core.dropCollection
+import org.springframework.data.mongodb.core.find
+import org.springframework.data.mongodb.core.insert
+import org.springframework.data.mongodb.core.query.Criteria
+import org.springframework.data.mongodb.core.query.Query
+import org.springframework.data.mongodb.core.query.isEqualTo
+import org.springframework.data.mongodb.core.query.regex
+import org.springframework.test.context.junit4.SpringRunner
+
+/**
+ * @author Christoph Strobl
+ */
+@RunWith(SpringRunner::class)
+@SpringBootTest
+class MongoDslTests {
+
+	@Autowired
+	lateinit var operations: MongoOperations
+
+	@Before
+	fun before() {
+		operations.dropCollection<Person>()
+	}
+
+	@Test
+	fun `simple type-safe find`() {
+
+		operations.insert<Person>().one(Person("Tyrion", "Lannister"))
+		operations.insert<Person>().one(Person("Cersei", "Lannister"))
+
+		val people = operations.find<Person>(Query(Person::firstname isEqualTo "Tyrion"))
+		assertThat(people).hasSize(1).extracting("firstname").containsOnly("Tyrion")
+	}
+
+	@Test
+	fun `more complex type-safe find`() {
+
+		operations.insert<Person>().one(Person("Tyrion", "Lannister"))
+		operations.insert<Person>().one(Person("Cersei", "Lannister"))
+
+		val people = operations.find<Person>(Query( //
+				Criteria().andOperator(Person::lastname isEqualTo "Lannister", Person::firstname regex "^Cer.*") //
+		))
+		assertThat(people).hasSize(1).extracting("firstname").containsOnly("Cersei")
+	}
+}