fix(username): sign in should work with post normalization (#4599)

Co-authored-by: Alex Yang <himself65@outlook.com>

packages/better-auth/src/plugins/username/index.ts

@@ -264,13 +264,13 @@ export const username = (options?: UsernameOptions) => {
 					}
 
 					const user = await ctx.context.adapter.findOne<
-						User & { username: string }
+						User & { username: string; displayUsername: string }
 					>({
 						model: "user",
 						where: [
 							{
 								field: "username",
-								value: username,
+								value: normalizer(username),
 							},
 						],
 					});
@@ -358,6 +358,7 @@ export const username = (options?: UsernameOptions) => {
 							email: user.email,
 							emailVerified: user.emailVerified,
 							username: user.username,
+							displayUsername: user.displayUsername,
 							name: user.name,
 							image: user.image,
 							createdAt: user.createdAt,

packages/better-auth/src/plugins/username/username.test.ts

@@ -269,6 +269,31 @@ describe("username", async (it) => {
 		expect(session?.user.username).toBe("custom_user");
 		expect(session?.user.displayUsername).toBe("Fancy Display Name");
 	});
+
+	it("should sign in with normalized username", async () => {
+		const { client } = await getTestInstance(
+			{
+				plugins: [username()],
+			},
+			{
+				clientOptions: {
+					plugins: [usernameClient()],
+				},
+			},
+		);
+		await client.signUp.email({
+			email: "normalized-username@email.com",
+			username: "Custom_User",
+			password: "test-password",
+			name: "test-name",
+		});
+		const res2 = await client.signIn.username({
+			username: "Custom_User",
+			password: "test-password",
+		});
+		expect(res2.data?.user.username).toBe("custom_user");
+		expect(res2.data?.user.displayUsername).toBe("Custom_User");
+	});
 });
 
 describe("username custom normalization", async (it) => {