distinct_on support (#124)

README.md

@@ -169,6 +169,33 @@ will generate the following GraphQL query variables:
 }
 ```
 
+and
+
+```jsx
+export const AddressList = () => (
+  <List
+    sort={{ field: 'city', order: 'DESC' }}
+    filter={{ distinct_on: 'city' }}
+  >
+    ...
+  </List>
+);
+```
+
+will generate the following GraphQL query variables:
+
+```json
+{
+  // ...
+  "order_by": {
+    "city": "desc"
+  },
+  "distinct_on": "city"
+}
+```
+
+Keep in mind that `distinct_on` must be used in conjunction with `order_by`, otherwise a `"distinct_on" columns must match initial "order_by" columns"` error will result. See more [here](https://hasura.io/docs/latest/queries/postgres/distinct-queries/#the-distinct_on-argument).
+
 ## Options
 
 ### Customize the Apollo client

src/buildVariables/buildGetListVariables.ts

@@ -25,6 +25,14 @@ export const buildGetListVariables: BuildGetListVariables =
     let { filter: filterObj = {} } = params;
     const { customFilters = [] } = params;
 
+    const distinctOnField = 'distinct_on';
+    /** Setting "distinct_on" to be the `filters` object attribute to be used inside RA
+     * and setting to a `distinct_on` variable
+     * and removing from the filter object
+     */
+    const { distinct_on = '' } = filterObj;
+    filterObj = omit(filterObj, [distinctOnField]);
+
     /**
      * Nested entities are parsed by CRA, which returns a nested object
      * { 'level1': {'level2': 'test'}}
@@ -173,5 +181,9 @@ export const buildGetListVariables: BuildGetListVariables =
       }
     }
 
+    if (distinct_on) {
+      result["distinct_on"] = distinct_on;
+    }
+
     return result;
   };