switch to a single model with a case, docs for it

models/reporting/xero/schema.yaml

@@ -1100,7 +1100,7 @@ models:
         data_type: text
         description: ""
 
-      - name: credit note_currency_iso_4217
+      - name: credit_note_currency_iso_4217
         data_type: character varying
         description: |
           The currency in which this line item amounts are defined.
@@ -1114,3 +1114,45 @@ models:
       - name: line_description
         data_type: text
         description: A text description of the line item.
+  - name: xero__net_fees
+    description: |
+      A summary aggregation table showing the total raw and net fees by month
+      and year.
+
+      Information comes from both Invoices and Credit notes.
+
+      Only documents in statuses ('PAID', 'AUTHORISED') are kept.
+
+      Net fees get computed by subtracting credited amounts from invoiced 
+      amounts.
+
+    columns:
+      - name: issued_year
+        data_type: numeric
+        description: The issuing year of the aggregated documents.
+
+      - name: issued_month
+        data_type: numeric
+        description: The issuing month of the aggregated documents.
+
+      - name: fee_category
+        data_type: text
+        description: | 
+          A fee category, one of out of booking_fees, listing_fees, 
+          verification_fees.
+
+      - name: fees_invoiced_in_gbp
+        data_type: numeric
+        description: The total sum of invoiced fees.
+
+      - name: fees_credited_in_gbp
+        data_type: numeric
+        description: The total sum of credited fees.
+
+      - name: net_fees_in_gbp
+        data_type: numeric
+        description: | 
+          Net fees are calculated by subtracting credited fees from invoiced
+          fees. It might be the case that net fees are negative in some month,
+          if it happens to be that we have given back to customers more than we
+          have invoiced them.