Support combining function and math expressions

Resolves #122

Author: CarsonBradshaw

packages/dynamodb-expressions/src/MathematicalExpression.ts

@@ -1,8 +1,9 @@
 import {AttributeBearingExpression} from "./AttributeBearingExpression";
 import {AttributePath} from "./AttributePath";
 import {ExpressionAttributes} from "./ExpressionAttributes";
+import {FunctionExpression} from "./FunctionExpression";
 
-export type MathematicalExpressionOperand = AttributePath|string|number;
+export type MathematicalExpressionOperand = AttributePath|FunctionExpression|string|number;
 
 const MATHEMATICAL_EXPRESSION_TAG = 'AmazonDynamoDbMathematicalExpression';
 const EXPECTED_TOSTRING = `[object ${MATHEMATICAL_EXPRESSION_TAG}]`;
@@ -21,9 +22,14 @@ export class MathematicalExpression implements AttributeBearingExpression {
 
     serialize(attributes: ExpressionAttributes) {
         const safeArgs = [this.lhs, this.rhs].map(
-            arg => AttributePath.isAttributePath(arg) || typeof arg === 'string'
-                ? attributes.addName(arg)
-                : attributes.addValue(arg)
+            arg => {
+                if (FunctionExpression.isFunctionExpression(arg)) {
+                    return arg.serialize(attributes);
+                }
+                return AttributePath.isAttributePath(arg) || typeof arg === 'string'
+                    ? attributes.addName(arg)
+                    : attributes.addValue(arg);
+            }
         );
         return `${safeArgs[0]} ${this.operator} ${safeArgs[1]}`;
     }