added head query to help users find earliest occurrence of data for a…

… job (#2)

influx_backend.go

@@ -136,6 +136,15 @@ func buildFluxQuery(bucket string, query *Query) (string, error) {
 		parts = append(parts, filterSubquery)
 	}
 
+	if query.Head != nil && query.Tail != nil {
+		return "", fmt.Errorf("head and tail cannot both be specified")
+	}
+
+	// add head subquery if included
+	if query.Head != nil {
+		parts = append(parts, fmt.Sprintf("limit(n:%d)", *query.Head))
+	}
+
 	// add tail subquery if included
 	if query.Tail != nil {
 		parts = append(parts, fmt.Sprintf("tail(n:%d)", *query.Tail))

influx_backend_test.go

@@ -48,14 +48,29 @@ func TestBuildFluxQuery(t *testing.T) {
 			Expect:     ``,
 			ShouldFail: true,
 		},
-		"StartAndEndRange": {
+		"StartEnd": {
+			Query: &Query{
+				Start: "-4h",
+				End:   "-2h",
+			},
+			Expect: `from(bucket:"mybucket") |> range(start:-4h,stop:-2h)`,
+		},
+		"StartEndTail": {
 			Query: &Query{
 				Start: "-4h",
 				End:   "-2h",
 				Tail:  intptr(3),
 			},
 			Expect: `from(bucket:"mybucket") |> range(start:-4h,stop:-2h) |> tail(n:3)`,
 		},
+		"StartEndHead": {
+			Query: &Query{
+				Start: "-4h",
+				End:   "-2h",
+				Head:  intptr(3),
+			},
+			Expect: `from(bucket:"mybucket") |> range(start:-4h,stop:-2h) |> limit(n:3)`,
+		},
 		"ExactFilter": {
 			Query: &Query{
 				Start: "-4h",
@@ -163,6 +178,10 @@ func TestBuildFluxBadQuery(t *testing.T) {
 		{
 			End: "); danger",
 		},
+		{
+			Head: intptr(3),
+			Tail: intptr(3),
+		},
 	}
 
 	for _, query := range testcases {

types.go

@@ -23,6 +23,7 @@ type Query struct {
 	Bucket *string           `json:"bucket,omitempty"`
 	Start  string            `json:"start,omitempty"`
 	End    string            `json:"end,omitempty"`
+	Head   *int              `json:"head,omitempty"`
 	Tail   *int              `json:"tail,omitempty"`
 	Filter map[string]string `json:"filter"`
 }