feat: add login mutation

packages/backend/src/graphql/mutations/login.ts

@@ -0,0 +1,30 @@
+import { GraphQLString, GraphQLNonNull } from 'graphql';
+import User from '../../models/user';
+import userType from '../types/user';
+
+type Params = {
+  email: string,
+  password: string
+}
+const loginResolver = async (params: Params) => {
+  const user = await User.query().findOne({
+    email: params.email,
+  });
+
+  if (user && await user.login(params.password)) {
+    return user;
+  }
+
+  throw new Error('User could not be found.')
+}
+
+const login = {
+  type: userType,
+  args: {
+    email: { type: GraphQLNonNull(GraphQLString) },
+    password: { type: GraphQLNonNull(GraphQLString) }
+  },
+  resolve: (_: any, params: any) => loginResolver(params)
+};
+
+export default login;