ErpNext Contract DocType

I'm Trying to Create Rent Contract DocType as the following: - Start Date (Date) - End Date (Date): To be calculated based of the (Duration of contract) - Duration of contract (Int) : Value in Months - Rent ( Currency) : The Total Amount of the Contract for the whole Duration - Rent Type (Master/DocType): Payment Period (Every 3 Month, Every 6 Months, Every 1 Year, ..) - TAX:( Sales Taxes and Charges Template) - Discount: (Percentage or Amount but not both) - Charges (Child Table): for Water, Gas, etc charges. it contains 2 fields: > Account(Account) > Amount(Currency) - Rent Invoices (Child Table): To generate Invoices based on (Rent Type) & (Duration of contract). it contains the following fields: > Date(date): Invoice Date > Invoice(Link): Link of the generated Invoice > Amount(Currency): The grand amount of the invoice > Generate (Button): To generate the invoice ** Generated invoice will be as follow - Invoice Date is Date from (Rent Invoices (Child Table)) - Tax will be fetch from (TAX) - The Charges Table in Invoice will fetch The (Charges (Child Table)) contents **Please note the I've mentioned only The Fields that is related to the project. All fields has been created, I require fetch, generate, post functions to be implemented which has to be done at [login to view URL] file. This DocType is part of an Custom App