Wrap Criteria is and regex comparison if necessary.

This commit wraps simple values and Patterns if to avoid creating invalid query objects.

spring-data-mongodb/src/main/java/org/springframework/data/mongodb/core/query/Criteria.java

@@ -945,8 +945,17 @@ protected Document getSingleCriteriaObject() {
 		Document queryCriteria = new Document();
 
 		if (!NOT_SET.equals(isValue)) {
-			queryCriteria.put(this.key, this.isValue);
-			queryCriteria.putAll(document);
+			if(document.isEmpty()) {
+				queryCriteria.put(this.key, this.isValue);
+			}
+			else {
+				if(isValue instanceof Pattern || isValue instanceof BsonRegularExpression) {
+					document.put("$regex", isValue);
+				} else {
+					document.put("$eq", isValue);
+				}
+				queryCriteria.put(this.key, document);
+			}
 		} else {
 			queryCriteria.put(this.key, document);
 		}

spring-data-mongodb/src/test/java/org/springframework/data/mongodb/core/query/CriteriaUnitTests.java

@@ -20,7 +20,9 @@
 import java.util.Arrays;
 import java.util.Collection;
 import java.util.Collections;
+import java.util.regex.Pattern;
 
+import org.bson.BsonRegularExpression;
 import org.bson.Document;
 import org.junit.Test;
 import org.springframework.data.geo.Point;
@@ -50,6 +52,44 @@ public void testSimpleCriteria() {
 		assertThat(c.getCriteriaObject()).isEqualTo("{ \"name\" : \"Bubba\"}");
 	}
 
+	@Test // GH-4850
+	public void testCombiningSimpleCriteria() {
+
+		Document expected = Document.parse("{ name : { $eq : 123, $type : ['long'] } }");
+
+		Criteria c = Criteria.where("name") //
+			.is(123) //
+			.type(Type.INT_64);
+
+		assertThat(c.getCriteriaObject()).isEqualTo(expected);
+
+		c = Criteria.where("name") //
+			.type(Type.INT_64)
+			.is(123);
+
+		assertThat(c.getCriteriaObject()).isEqualTo(expected);
+	}
+
+	@Test // GH-4850
+	public void testCombiningBsonRegexCriteria() {
+
+		Criteria c = Criteria.where("name")
+			.regex(new BsonRegularExpression("^spring$"))
+			.type(Type.INT_64);
+
+		assertThat(c.getCriteriaObject()).isEqualTo(Document.parse("{ name : { $regex : RegExp('^spring$'), $type : ['long'] } }"));
+	}
+
+	@Test // GH-4850
+	public void testCombiningRegexCriteria() {
+
+		Criteria c = Criteria.where("name")
+			.regex("^spring$")
+			.type(Type.INT_64);
+
+		assertThat(c.getCriteriaObject()).hasEntrySatisfying("name.$regex", it -> assertThat(it).isInstanceOf(Pattern.class));
+	}
+
 	@Test
 	public void testNotEqualCriteria() {
 		Criteria c = new Criteria("name").ne("Bubba");