feat: Enable discount accounting

[GangaManoj]

Problem:

Currently any item or addition discount applied is accounted in the Income account itself and there is no way to account for discounts in a separate account head. The only workaround possible for now is to use Item Tax templates with negative rate but there is no way to apply fixed amount discount (has to be done manually by back calculating rate) and add that rate in Item Tax Template

Fix:

• Add a checkbox to Enable Discount Accounting in Accounts Settings.

• If the aforementioned box is checked, the Discount Account field in the Items table for Sales and Purchase Invoices will be made visible using a Property Setter.

• This will be allow you to create GL Entries in the following manner:

For Sales Invoice

For Purchase Invoice

• Additionally, you could enter a Default Discount Account for an Item, which will be fetched automatically while creating the Invoice. The visibility for this field is also dependant on the Enable Discount Accounting checkbox in Accounts Settings.

• If Enable Discount Account is checked, the Additional Discount Account field will also be made visible. On submitting the invoice, GL Entries will be made against the Additional Discount Account for the Additional Discount Amount, as show in the given example.

Example

Item cost = Rs 100
Tax Amount = Rs 20
Discount Percentage = 20%
Therefore, Discount Amount = Rs 24

For Sales Invoice

Before:

After:

For Purchase Invoice

Before:

After:

Docs

• docs: Add docs for Discount Account erpnext_documentation#361
• docs: Update Accounts Settings docs erpnext_documentation#360

Testing Info

1. Open Accounts Settings.
2. Check the Enable Discount Accounting box.
3. Create a new Item/open an existing Item and go to the Sales Defaults section.
4. Enter a Default Discount Account.
5. Create a Sales Invoice for that Item.
6. Enter Discount Amount in the Items table.
7. The Discount Account will be the Item's Default Discount Account, but this can be changed.
8. Save and Submit.
9. Check the GL Entries created and see if they're similar to ones in the image provided above.

[deepeshgarg007]

• One more discount account field at parent level needs to be added if additional discounts are applied. If additional discount is applied on Grand Total then tax GL Entries should be made for full amount instead of post discount amount

• Add test cases

[coveralls]

Coverage remained the same at 42.826% when pulling b85d301 on GangaManoj:enable-discount-accounting into 0b320b5 on frappe:develop.

[nabinhait]

Great description for PR 👍

[GangaManoj]

@nabinhait Thanks!

[pruthvi145]

Why you did None or? Isn't it always run the second part?