Add data to a JSON model in SAPUI5

I am currently learning SAPUI5 and wanted to create a simple form as an exercise. Unfortunately I can't manage to create a new entry in my JSON model. When I try to execute my code, I don't get an error message and the entry is not copied to the JSON file.

I have tried the following:

EmployeeForm.view.xml

<mvc:View xmlns:core="sap.ui.core"
controllerName="sap.ui.test.controller.EmployeeForm"
    xmlns="sap.m"
    xmlns:form="sap.ui.layout.form"
    xmlns:mvc="sap.ui.core.mvc">
    <Page
        showNavButton="true"
        navButtonPress=".onNavBack">
        <Panel headerText="{i18n>employeeFormHeaderText}">
            <form:SimpleForm editable="true" layout="ColumnLayout">
                <Label text="First Name"/>
                <Input value="{/first_name}" width="200px" />
                <Label text="Last Name"/>
                <Input value="{/last_name}" width="200px" />
                <Label text="Email"/>
                <Input value="{/email}" width="200px"/>
                <Label text="Gender"/>
                <Select width="200px">
                    <items>
                        <core:Item text="Male" key="male"></core:Item>
                        <core:Item text="Female" key="female"></core:Item>
                        <core:Item text="Other" key="other"></core:Item>
                    </items>
                </Select>
                <Label text="IP Address"/>
                <Input value="{/ip_address}" width="200px"/>
            </form:SimpleForm>
            <VBox width="100%" alignItems="Center">
                <Button 
                icon="sap-icon://save"
                text="{i18n>employeeFormButtonText}"
                press=".onSaveEmployee"></Button>
            </VBox>
        </Panel>
    </Page>
</mvc:View>

EmployeeForm.controller.js

sap.ui.define([
    "sap/ui/core/mvc/Controller",
    "sap/ui/core/routing/History",
    "sap/ui/model/json/JSONModel"
], function (Controller, History, JSONModel) {
    "use strict";

    return Controller.extend("sap.ui.test.controller.EmployeeForm", {
        onInit: function () {
            var model = this.getOwnerComponent().getModel("employee");
            var modelLength = model.oData.Employees.length + 1;
            var oModel = new JSONModel({
                id: modelLength,
                first_name: "",
                last_name: "",
                email: "",
                gender: "",
                ip_address: "",
                status: "work"
            });
            this.getView().setModel(oModel);
        },
        onSaveEmployee: function () {
            // get item
            var newItem = this.getView().getModel().oData;
            // get model
            var oModel = this.getOwnerComponent().getModel("employee");
            // add item to model
            oModel.setProperty("/Employees", oModel.getProperty("/Employees").concat(newItem));
        }
    });
});

manifest.json

...
"models": {
     "employee": {
         "type": "sap.ui.model.json.JSONModel",
         "uri": "Employees.json"
     }
},
...

The JSONModel only uses the data from the JSON file as initial data, it will never write back any changes made after the initial load.

All changes done (if in two way binding) during the use of the application should be visible on the JSONModel object but they will be lost when refreshing since no writing directly on the JSON file will be done to make those changes permanent.