refactoring, support for storing feature photos

* errors put into separate package
* added table to store feature photos
* added API endpoints for serving photos
* POST /data extended to accept photos using a multipart/form-data body

models/models.go

@@ -1,11 +1,16 @@
 package models
 
-import geojson "github.com/paulmach/go.geojson"
+import (
+	"io"
+
+	geojson "github.com/paulmach/go.geojson"
+)
 
 // core objects
 
-type UserID int
-type FeatureID int
+type UserID int64
+type FeatureID int64
+type FeaturePhotoID int64
 
 type User struct {
 	ID   UserID `json:"id"`
@@ -13,20 +18,29 @@ type User struct {
 }
 
 type Feature struct {
+	// ID is an ID of the feature.
+	// When the feature is submitted by a client for creation (i.e. in Update.Create) it is considered a 'local' ID which must be unique across all submitted features.
 	ID          FeatureID        `json:"id"`
 	OwnerID     *UserID          `json:"owner_id"`
 	Name        string           `json:"name"`
 	Description *string          `json:"description"`
 	Category    *string          `json:"category"`
 	Geometry    geojson.Geometry `json:"geometry"`
+	// PhotoIDs contains a list IDs of photos associated with the feature.
+	// When the feature is retrieved from the server, the IDs can be used to retrieve the photos.
+	// When the feature is submitted by a client (created or updated, i.e. in Update.Create or Update.Update), this list is ignored (as photos are managed through Update.CreatePhotos and Update.DeletePhotos fields).
+	PhotoIDs []FeaturePhotoID `json:"photo_ids"`
 }
 
 // transport objects
 
 type Update struct {
-	Create []Feature   `json:"create"`
-	Update []Feature   `json:"update"`
-	Delete []FeatureID `json:"delete"`
+	Create        []Feature              `json:"create"`
+	CreatedPhotos map[FeatureID][]string `json:"created_photos"`
+	AddPhotos     map[FeatureID][]string `json:"add_photos"`
+	Update        []Feature              `json:"update"`
+	Delete        []FeatureID            `json:"delete"`
+	DeletePhotos  []FeaturePhotoID       `json:"delete_photos"`
 }
 
 type HandshakeChallenge struct {
@@ -44,3 +58,9 @@ type Data struct {
 	Users    []User    `json:"users"`
 	Features []Feature `json:"features"`
 }
+
+type Photo struct {
+	ContentType string
+	File        io.ReadCloser
+	Size        int64
+}