fix: avoid schema field mutation when passing selectedFields opt

src/bigquery.ts

@@ -362,17 +362,17 @@
   private _universeDomain: string;
   private _enableQueryPreview: boolean;
 
   createQueryStream(options?: Query | string): ResourceStream<RowMetadata> {
     // placeholder body, overwritten in constructor
     return new ResourceStream<RowMetadata>({}, () => {});
   }
 
   getDatasetsStream(options?: GetDatasetsOptions): ResourceStream<Dataset> {
     // placeholder body, overwritten in constructor
     return new ResourceStream<Dataset>({}, () => {});
   }
 
   getJobsStream(options?: GetJobsOptions): ResourceStream<Job> {
     // placeholder body, overwritten in constructor
     return new ResourceStream<Job>({}, () => {});
   }
@@ -588,14 +588,16 @@
       parseJSON?: boolean;
     }
   ) {
+    // copy schema fields to avoid mutation when filtering selected fields
+    let schemaFields = schema.fields ? [...schema.fields] : [];
     if (options.selectedFields && options.selectedFields!.length > 0) {
       const selectedFieldsArray = options.selectedFields!.map(c => {
         return c.split('.');
       });
 
       const currentFields = selectedFieldsArray.map(c => c.shift());
       //filter schema fields based on selected fields.
-      schema.fields = schema.fields?.filter(
+      schemaFields = schemaFields.filter(
         field =>
           currentFields
             .map(c => c!.toLowerCase())
@@ -610,7 +612,7 @@
 
     function mergeSchema(row: TableRow) {
       return row.f!.map((field: TableRowField, index: number) => {
-        const schemaField = schema.fields![index];
+        const schemaField = schemaFields[index];
         let value = field.v;
         if (schemaField.mode === 'REPEATED') {
           value = (value as TableRowField[]).map(val => {
@@ -1569,7 +1571,7 @@
     const parameterMode = is.array(params) ? 'positional' : 'named';
     const queryParameters: bigquery.IQueryParameter[] = [];
     if (parameterMode === 'named') {
       const namedParams = params as {[param: string]: any};
       for (const namedParameter of Object.getOwnPropertyNames(namedParams)) {
         const value = namedParams[namedParameter];
         let queryParameter;
@@ -2212,7 +2214,7 @@
 
       options = extend({job}, queryOpts, options);
       if (res && res.jobComplete) {
         let rows: any = [];
         if (res.schema && res.rows) {
           rows = BigQuery.mergeSchemaWithRows_(res.schema, res.rows, {
             wrapIntegers: options.wrapIntegers || false,

src/table.ts

@@ -1830,6 +1830,10 @@ class Table extends ServiceObject {
     delete options.wrapIntegers;
     const parseJSON = options.parseJSON ? options.parseJSON : false;
     delete options.parseJSON;
+    const selectedFields = options.selectedFields
+      ? options.selectedFields.split(',')
+      : [];
+    delete options.selectedFields;
     const onComplete = (
       err: Error | null,
       rows: TableRow[] | null,
@@ -1841,10 +1845,8 @@ class Table extends ServiceObject {
         return;
       }
       rows = BigQuery.mergeSchemaWithRows_(this.metadata.schema, rows || [], {
-        wrapIntegers: wrapIntegers,
-        selectedFields: options.selectedFields
-          ? options.selectedFields!.split(',')
-          : [],
+        wrapIntegers,
+        selectedFields,
         parseJSON,
       });
       callback!(null, rows, nextQuery, resp);

test/table.ts

@@ -2189,6 +2189,56 @@ describe('BigQuery/Table', () => {
       });
     });
 
+    it('should return selected fields after consecutive calls', done => {
+      const callSequence = [
+        {
+          selectedFields: ['age'],
+          rows: [{f: [{v: 40}]}],
+          expected: [{age: 40}],
+        },
+        {
+          selectedFields: ['name', 'address'],
+          rows: [{f: [{v: 'John'}, {v: '1234 Fake St, Springfield'}]}],
+          expected: [{name: 'John', address: '1234 Fake St, Springfield'}],
+        },
+        {
+          selectedFields: ['age'],
+          rows: [{f: [{v: 50}]}],
+          expected: [{age: 50}],
+        },
+      ];
+      const schema = {
+        fields: [
+          {name: 'name', type: 'string'},
+          {name: 'age', type: 'INTEGER'},
+          {name: 'address', type: 'string'},
+        ],
+      };
+
+      table.metadata = {schema};
+
+      sandbox.restore();
+
+      for (const [i, call] of callSequence.entries()) {
+        table.request = (
+          reqOpts: DecorateRequestOptions,
+          callback: Function
+        ) => {
+          callback(null, {rows: call.rows});
+        };
+        table.getRows(
+          {selectedFields: call.selectedFields.join(',')},
+          (err: Error, rows: {}) => {
+            assert.ifError(err);
+            assert.deepStrictEqual(rows, call.expected);
+            if (i === callSequence.length - 1) {
+              done();
+            }
+          }
+        );
+      }
+    });
+
     it('should return selected fields from nested objects', done => {
       const selectedFields = 'objects.nested_object.nested_property_1';
       const rows = [