Detect tag ausence when parsing JSON to Item

src/Item.zig

@@ -65,11 +65,11 @@ pub fn toJson(self: Self) json.Obj {
     // Add id (i64|null)
     jobj.objectAdd("id", if (self.id) |id| &json.Obj.newInt64(id) else null);
 
-    var jtags = json.Obj.newObject();
-    jobj.objectAdd("tags", &jtags);
-
     // Add tags only if they're initialized
     if (self.tags) |tags| {
+        var jtags = json.Obj.newObject();
+        jobj.objectAdd("tags", &jtags);
+
         for (tags) |tag_i| {
             const jtag = if (tag_i.value) |value|
                 &json.Obj.newString(value)
@@ -78,26 +78,37 @@ pub fn toJson(self: Self) json.Obj {
 
             jtags.objectAdd(tag_i.name, jtag);
         }
+    } else {
+        // If nothing is found, set it to null
+        // just to make it more explicit
+        jobj.objectAdd("tags", null);
     }
 
     return jobj;
 }
 
 pub fn fromJson(jobj: json.Obj, allocator: std.mem.Allocator) !Self {
-    var jtags = jobj.objectGet("tags");
-    defer jtags.deinit();
+    // Try to assemble a Item object from a JSON string
+    // An item could be just an id, just tags or both.
 
-    const len = @intCast(usize, jtags.objectLen());
-    var tags = try allocator.alloc(Tag, len);
+    var opt_jtags = jobj.objectGet("tags") catch null;
+    var tags: ?[]Tag = null;
 
-    var iter = jtags.objectGetIterator();
+    if (opt_jtags) |*jtags| {
+        defer jtags.deinit();
+        const len = @intCast(usize, jtags.objectLen());
+        tags = try allocator.alloc(Tag, len);
 
-    var i: usize = 0;
-    while (iter.next()) |*tag| {
-        tags[i].name = try allocator.dupeZ(u8, tag.key);
-        tags[i].value = if (tag.value) |*value| try allocator.dupeZ(u8, value.getString()) else null;
+        var iter = jtags.objectGetIterator();
 
-        i += 1;
+        var i: usize = 0;
+        while (iter.next()) |*tag| {
+            // Whe know it's not null at this point
+            tags.?[i].name = try allocator.dupeZ(u8, tag.key);
+            tags.?[i].value = if (tag.value) |*value| try allocator.dupeZ(u8, value.getString()) else null;
+
+            i += 1;
+        }
     }
 
     return Self{

src/json.zig

@@ -1,10 +1,21 @@
-const c = @cImport({
+pub const c = @cImport({
     @cInclude("json.h");
 });
 const std = @import("std");
 
+const Type = enum(c.json_type) {
+    @"null" = c.json_type_null,
+    boolean = c.json_type_boolean,
+    double = c.json_type_double,
+    int = c.json_type_int,
+    object = c.json_type_object,
+    array = c.json_type_array,
+    string = c.json_type_string,
+};
+
 pub const Obj = struct {
-    obj: *c.json_object,
+    // Option because a null value is treated as the C value NULL in json-c
+    obj: ?*c.json_object,
 
     // TODO: Port type attribute to a zig enum and assign it upon creation
 
@@ -31,6 +42,15 @@ pub const Obj = struct {
         return Obj{ .obj = c.json_tokener_parse(s).? };
     }
 
+    const TypeError = error {
+        TypeMismatch, // Tried to use a function on the wrong JSON format
+    };
+
+    pub fn getType(self: Obj) Type {
+        //std.debug.print("{}\n", .{c.json_object_get_type(self.obj)});
+        return @intToEnum(Type, c.json_object_get_type(self.obj));
+    }
+
     // Not *const Obj because the json_object retains ownership of the string
     pub fn getString(self: *Obj) []const u8 {
         // TODO: Check type to not allow other types
@@ -51,13 +71,27 @@ pub const Obj = struct {
     // ** Object functions **
     /////////////////////////////////
 
-    // TODO: Create an error for type checking.
+    const GetError = error {
+        TypeMismatch, // Not an object/array
+        KeyNotFound, // Key or ID not present in the object/array
+    };
 
-    pub fn objectGet(self: *const Obj, key: [*c]const u8) Obj {
+    pub fn objectGet(self: *const Obj, key: [*c]const u8) GetError!Obj {
         // TODO: Check type and null return as errors
         var obj: ?*c.json_object = undefined;
-        _ = c.json_object_object_get_ex(self.obj, key, &obj);
-        return Obj{ .obj = c.json_object_get(obj).? };
+        const succ = c.json_object_object_get_ex(self.obj, key, &obj);
+
+        // Everything went alright
+        if (obj) |ret| {
+            return Obj{ .obj = c.json_object_get(ret).? };
+        } else if (obj == null and succ != 0){
+            // The actual value is just null
+            return Obj{ .obj = null };
+        } else if (self.getType() != Type.object){
+            return GetError.TypeMismatch;
+        } else {
+            return GetError.KeyNotFound;
+        }
     }
 
     pub fn objectLen(self: *const Obj) c_int {

src/main.zig

@@ -31,7 +31,19 @@ pub fn main() !void {
 
     try item.persist(&db);
 
-    var jobj = item.toJson();
+    // ----------
+
+    const jsonText2: [:0]const u8 =
+        \\{ "id" : 1 }
+    ;
+
+    var jsonItem2 = json.Obj.newFromString(jsonText2);
+    defer jsonItem2.deinit();
+
+    var item2 = try Item.fromJson(jsonItem2, allocator);
+    defer item2.deinit();
+
+    var jobj = item2.toJson();
     defer jobj.deinit();
 
     std.debug.print("{s}\n", .{jobj.toString()});