[Lens] Error on array values in math

src/plugins/expressions/common/expression_functions/specs/math_column.ts

@@ -69,25 +69,40 @@ export const mathColumn: ExpressionFunctionDefinition<
       return id === args.id;
     });
     if (existingColumnIndex > -1) {
-      throw new Error('ID must be unique');
+      throw new Error(
+        i18n.translate('expressions.functions.mathColumn.uniqueIdError', {
+          defaultMessage: 'ID must be unique',
+        })
+      );
     }
 
     const newRows = input.rows.map((row) => {
-      return {
-        ...row,
-        [args.id]: math.fn(
-          {
-            type: 'datatable',
-            columns: input.columns,
-            rows: [row],
-          },
-          {
-            expression: args.expression,
-            onError: args.onError,
-          },
-          context
-        ),
-      };
+      const result = math.fn(
+        {
+          type: 'datatable',
+          columns: input.columns,
+          rows: [row],
+        },
+        {
+          expression: args.expression,
+          onError: args.onError,
+        },
+        context
+      );
+
+      if (Array.isArray(result)) {
+        if (result.length === 1) {
+          return { ...row, [args.id]: result[0] };
+        }
+        throw new Error(
+          i18n.translate('expressions.functions.mathColumn.arrayValueError', {
+            defaultMessage: 'Cannot perform math on array values at {name}',
+            values: { name: args.name },
+          })
+        );
+      }
+
+      return { ...row, [args.id]: result };
     });
     const type = newRows.length ? getType(newRows[0][args.id]) : 'null';
     const newColumn: DatatableColumn = {

src/plugins/expressions/common/expression_functions/specs/tests/math_column.test.ts

@@ -34,6 +34,30 @@ describe('mathColumn', () => {
     });
   });
 
+  it('extracts a single array value, but not a multi-value array', () => {
+    const arrayTable = {
+      ...testTable,
+      rows: [
+        {
+          name: 'product1',
+          time: 1517842800950, // 05 Feb 2018 15:00:00 GMT
+          price: [605, 500],
+          quantity: [100],
+          in_stock: true,
+        },
+      ],
+    };
+    const args = {
+      id: 'output',
+      name: 'output',
+      expression: 'quantity',
+    };
+    expect(fn(arrayTable, args).rows[0].output).toEqual(100);
+    expect(() => fn(arrayTable, { ...args, expression: 'price' })).toThrowError(
+      `Cannot perform math on array values`
+    );
+  });
+
   it('handles onError', () => {
     const args = {
       id: 'output',

src/plugins/expressions/common/expression_types/get_type.test.ts

@@ -30,6 +30,7 @@ describe('getType()', () => {
   });
 
   test('throws if object has no .type property', () => {
+    expect(() => getType([])).toThrow();
     expect(() => getType({})).toThrow();
     expect(() => getType({ _type: 'foo' })).toThrow();
     expect(() => getType({ tipe: 'foo' })).toThrow();

src/plugins/expressions/common/expression_types/get_type.ts

@@ -8,6 +8,9 @@
 
 export function getType(node: any) {
   if (node == null) return 'null';
+  if (Array.isArray(node)) {
+    throw new Error('Unexpected array value encountered.');
+  }
   if (typeof node === 'object') {
     if (!node.type) throw new Error('Objects must have a type property');
     return node.type;

x-pack/plugins/lens/public/indexpattern_datasource/operations/definitions/formula/math.tsx

@@ -48,7 +48,7 @@ export const mathOperation: OperationDefinition<MathIndexPatternColumn, 'managed
         function: 'mathColumn',
         arguments: {
           id: [columnId],
-          name: [columnId],
+          name: [column.label],
           expression: [astToString(column.params.tinymathAst)],
           onError: ['null'],
         },