Add invoice discount to invoice_double_entry [resolve #59]

[SellJamHere]

Are you a current Fivetran customer?
Yes. James Heller from Apartment 304.

What change(s) does this PR introduce?
This change adds invoice discounts to the int_quickbooks__invoice_double_entry table, and calculates the expected Accounts Receivable amount (sum of sales/service line items less discount), instead of creating an A/R record for each sales/service line item in the invoice.

Did you update the CHANGELOG?

• Yes

Not yet. I would like to review the changes with the team.

Does this PR introduce a breaking change?

• Yes (please provide breaking change details below.)
• No (please provide an explanation as to how the change is non-breaking below.)

This change will generate correct invoice amounts for users using invoice discounts.

Did you update the dbt_project.yml files with the version upgrade (please leverage standard semantic versioning)? (In both your main project and integration_tests)

• Yes

Not yet. I would like to review the changes with the team.

Is this PR in response to a previously created Bug or Feature Request

• Yes, Issue/Feature [[Bug] Invoice discounts aren't included as line items in the general ledger #59]
• No

How did you test the PR changes?

• CircleCi
• Local (please provide additional testing details below)
• Fivetran

I tested the feature using a Fivetran Transformation on a Databricks destination. I'm happy to take on further testing and validation if the team thinks the query changes can be merged in.

Select which warehouse(s) were used to test the PR

• BigQuery
• Redshift
• Snowflake
• Postgres
• Databricks
• Other (provide details below)

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We are so excited you decided to contribute to the Fivetran community dbt package! We continue to work to improve the packages and would greatly appreciate your feedback on our existing dbt packages or what you'd like to see next.

[fivetran-joemarkiewicz]

@SellJamHere thanks again for opening this PR. I am currently taking a look and had a quick question.

I see you do a lot of work to separate out the invoice discounts from the normal invoice lines. However, would it work instead to still keep the invoice and invoice discounts together, but have a case when statement for amount to appropriately convey the discount as a subtraction from the total invoice amount? Essentially I am thinking we could possible use the following in the double entry model for invoices:

invoice_join as (

    select
        invoices.invoice_id as transaction_id,
        invoices.source_relation,
        invoice_lines.index, 
        invoices.transaction_date as transaction_date,
        case when invoices.total_amount != 0 then invoice_lines.amount
            when coalesce(invoices.total_amount,0) == 0 then invoices.total_amount
            when invoice_lines.discount_account_id is not null then amount * -1
        end as amount,

What are your thoughts?

[fivetran-joemarkiewicz]

@SellJamHere thanks again for opening this PR! This gives us a great start into considering discounts properly in the invoice double entry and downstream models!

There are likely a few changes I will make to your updates once I merge this into a staging branch. I highly recommend following the corresponding PRs to give the updates a try on your end to confirm your initial issue is addressed.

Thanks again!