Add Min/Max find options (#201)

* Add Min/Max find options

* Add simple test for Min/Max find methods

Author: bozaro

session.go

@@ -3455,6 +3455,34 @@ func (q *Query) Sort(fields ...string) *Query {
 	return q
 }
 
+// Specify a $min value to specify the inclusive lower bound for a specific index in order to constrain the results of
+// Find(). The $min specifies the lower bound for all keys of a specific index in order.
+//
+// Relevant documentation:
+//
+//     https://docs.mongodb.com/manual/reference/operator/meta/min/
+//
+func (q *Query) Min(min interface{}) *Query {
+	q.m.Lock()
+	q.op.options.Min = min
+	q.m.Unlock()
+	return q
+}
+
+// Specify a $max value to specify the exclusive upper bound for a specific index in order to constrain the results of
+// Find(). The $max specifies the upper bound for all keys of a specific index in order.
+//
+// Relevant documentation:
+//
+//     https://docs.mongodb.com/manual/reference/operator/meta/max/
+//
+func (q *Query) Max(max interface{}) *Query {
+	q.m.Lock()
+	q.op.options.Max = max
+	q.m.Unlock()
+	return q
+}
+
 // Collation allows to specify language-specific rules for string comparison,
 // such as rules for lettercase and accent marks.
 // When specifying collation, the locale field is mandatory; all other collation
@@ -3787,6 +3815,8 @@ func prepareFindOp(socket *mongoSocket, op *queryOp, limit int32) bool {
 		MaxTimeMS:       op.options.MaxTimeMS,
 		MaxScan:         op.options.MaxScan,
 		Hint:            op.options.Hint,
+		Min:             op.options.Min,
+		Max:             op.options.Max,
 		Comment:         op.options.Comment,
 		Snapshot:        op.options.Snapshot,
 		Collation:       op.options.Collation,

session_test.go

@@ -1634,6 +1634,37 @@ func (s *S) TestViewWithCollation(c *C) {
 	c.Assert(docs[1].Nm, Equals, "CaSe")
 }
 
+func (s *S) TestFindWithMinMax(c *C) {
+	if !s.versionAtLeast(3, 4) {
+		c.Skip("depends on mongodb 3.4+")
+	}
+	// CreateView has to be run against mongos
+	session, err := mgo.Dial("localhost:40001")
+	c.Assert(err, IsNil)
+	defer session.Close()
+
+	db := session.DB("mydb")
+
+	coll := db.C("mycoll")
+
+	for i := 0; i < 4; i++ {
+		err = coll.Insert(bson.M{"_id": i, "nm": "a"})
+		c.Assert(err, IsNil)
+		err = coll.Insert(bson.M{"_id": fmt.Sprintf("x%d", i), "nm": "b"})
+		c.Assert(err, IsNil)
+	}
+
+	ids := []interface{}{}
+	iter := coll.Find(nil).Sort("_id").Min(bson.M{"_id": 2}).Max(bson.M{"_id": "x2"}).Iter()
+	var doc bson.M
+	for iter.Next(&doc) {
+		ids = append(ids, doc["_id"])
+	}
+	c.Assert(iter.Err(), IsNil)
+
+	c.Assert(ids, DeepEquals, []interface{}{2, 3, "x0", "x1"})
+}
+
 func (s *S) TestCountQuery(c *C) {
 	session, err := mgo.Dial("localhost:40001")
 	c.Assert(err, IsNil)

socket.go

@@ -89,6 +89,8 @@ type queryWrapper struct {
 	Query          interface{} `bson:"$query"`
 	OrderBy        interface{} `bson:"$orderby,omitempty"`
 	Hint           interface{} `bson:"$hint,omitempty"`
+	Min            interface{} `bson:"$min,omitempty"`
+	Max            interface{} `bson:"$max,omitempty"`
 	Explain        bool        `bson:"$explain,omitempty"`
 	Snapshot       bool        `bson:"$snapshot,omitempty"`
 	ReadPreference bson.D      `bson:"$readPreference,omitempty"`