Add mongo optimization to convert $in/$nin into $eq/$ne (for special …

…case)

When argument to $in/$nin call is a list with **single** element convert it
to $eq/$ne. For some reason it is slow on some mongo versions.

Update tests to validate this behaviour

criteria/common/test/org/immutables/criteria/personmodel/AbstractPersonTest.java

@@ -277,7 +277,10 @@ public void basic() {
     check(person.fullName.is("John")
             .age.is(22)).hasSize(1);
     check(person.fullName.is("_MISSING_")).empty();
+    check(person.fullName.in(Collections.emptyList())).empty();
+    check(person.fullName.in(Collections.singleton("John"))).hasSize(1);
     check(person.fullName.in("John", "test2")).hasSize(1);
+    check(person.fullName.notIn(Collections.singleton("John"))).empty();
     check(person.fullName.notIn("John", "test2")).empty();
 
     // true / false

criteria/common/test/org/immutables/criteria/typemodel/StringTemplate.java

@@ -212,6 +212,31 @@ protected void optional() {
     values(string.optional.is("")).isEmpty();
   }
 
+  /**
+   * cases for {@code foo in [1]} or {@code foo not in [1]}.
+   * When list has just one element
+   */
+  @Test
+  void inNotIn_singleElement() {
+    repository.insert(generator.get().withId("id1").withValue("value1"));
+    repository.insert(generator.get().withId("id2").withValue(""));
+
+    // for id
+    ids(string.id.in(Collections.emptyList())).isEmpty();
+    ids(string.id.in(Collections.singleton("id1"))).isOf("id1");
+    ids(string.id.notIn(Collections.singleton("id1"))).isOf("id2");
+    ids(string.id.notIn(Collections.singleton("id2"))).isOf("id1");
+    ids(string.id.notIn(Collections.emptyList())).hasContentInAnyOrder("id1", "id2");
+
+    // for value
+    ids(string.value.in(Collections.emptyList())).isEmpty();
+    ids(string.value.in(Collections.singleton("value1"))).isOf("id1");
+    ids(string.value.notIn(Collections.singleton("value1"))).isOf("id2");
+    ids(string.value.notIn(Collections.singleton(""))).isOf("id1");
+    ids(string.value.notIn(Collections.emptyList())).hasContentInAnyOrder("id1", "id2");
+
+  }
+
   /**
    * validate {@code one() / oneOrNone() / exists()} methods
    */

criteria/mongo/src/org/immutables/criteria/mongo/FindVisitor.java

@@ -123,6 +123,12 @@ private Bson binaryCall(Call call) {
     if (op == Operators.IN || op == Operators.NOT_IN) {
       final List<Object> values = Visitors.toConstant(call.arguments().get(1)).values();
       Preconditions.checkNotNull(values, "not expected to be null for %s", op);
+      if (values.size() == 1) {
+        // optimization: convert IN, NIN (where argument is a list with single element) into EQ / NE
+        Operators newOperator = op == Operators.IN ? Operators.EQUAL : Operators.NOT_EQUAL;
+        Call newCall = Expressions.call(newOperator, call.arguments().get(0), Expressions.constant(values.get(0)));
+        return binaryCall(newCall);
+      }
       return op == Operators.IN ? Filters.in(field, values) : Filters.nin(field, values);
     }