implement flatMap operation for query streams

lib/wallet/query.ts

@@ -38,6 +38,7 @@ export interface QueryStream<T> {
                keyFn: (obj: any) => any): QueryStream<[T,S]>;
   filter(f: (any) => boolean): QueryStream<T>;
   reduce<S>(f: (v: T, acc: S) => S, start?: S): Promise<S>;
+  flatMap(f: (T) => T[]): QueryStream<T>;
 }
 
 
@@ -68,6 +69,10 @@ abstract class QueryStreamBase<T> implements QueryStream<T> {
     this.root = root;
   }
 
+  flatMap(f: (T) => T[]): QueryStream<T> {
+    return new QueryStreamFlatMap(this, f);
+  }
+
   indexJoin<S>(storeName: string,
                indexName: string,
                key: any): QueryStream<[T,S]> {
@@ -117,6 +122,31 @@ class QueryStreamFilter<T> extends QueryStreamBase<T> {
         return;
       }
       if (this.filterFn(value)) {
+        f(false, value, tx);
+      }
+    });
+  }
+}
+
+
+class QueryStreamFlatMap<T> extends QueryStreamBase<T> {
+  s: QueryStreamBase<T>;
+  flatMapFn;
+
+  constructor(s: QueryStreamBase<T>, flatMapFn) {
+    super(s.root);
+    this.s = s;
+    this.flatMap = flatMapFn;
+  }
+
+  subscribe(f) {
+    this.s.subscribe((isDone, value, tx) => {
+      if (isDone) {
+        f(true, undefined, tx);
+        return;
+      }
+      let values = this.flatMapFn(value);
+      for (let v in values) {
         f(false, value, tx)
       }
     });