GraphDbAccessor::Transfer added

Summary: A helper function added for transferring graph elements into some GraphDbAccessor.

Reviewers: mislav.bradac, buda, teon.banek

Reviewed By: mislav.bradac

Subscribers: pullbot

Differential Revision: https://phabricator.memgraph.io/D581

src/database/graph_db_accessor.hpp

@@ -298,16 +298,45 @@ class GraphDbAccessor {
   }
 
   /**
-   * Adds an index for the given (label, property) and populates
-   * it with existing vertices that belong to it.
+   * Creates and returns a new accessor that represents the same graph element
+   * (node / version) as the given `accessor`, but in this `GraphDbAccessor`.
    *
-   * You should never call BuildIndex on a GraphDbAccessor
-   * (transaction) on which new vertices have been inserted or
-   * existing ones updated. Do it in a new accessor instead.
+   * It is possible that the given `accessor` graph element is not visible in
+   * this `GraphDbAccessor`'s transaction.  If that is the case, a `nullopt` is
+   * returned.
    *
-   * Build index throws if an index for the given
-   * (label, property) already exists (even if it's being built
-   * by a concurrent transaction and is not yet ready for use).
+   * The returned accessor does NOT have the same `current_` set as the given
+   * `accessor`. It has default post-construction `current_` set (`old` if
+   * available, otherwise `new`).
+   *
+   * @param accessor The [Vertex/Edge]Accessor whose underlying graph
+   *  element we want in this GraphDbAccessor.
+   * @return See above.
+   * @tparam TAccessor Either VertexAccessor or EdgeAccessor
+   */
+  template <typename TAccessor>
+  std::experimental::optional<TAccessor> Transfer(const TAccessor &accessor) {
+    if (accessor.db_accessor_ == this)
+      return std::experimental::make_optional(accessor);
+
+    TAccessor accessor_in_this(*accessor.vlist_, *this);
+    if (accessor_in_this.current_)
+      return std::experimental::make_optional(std::move(accessor_in_this));
+    else
+      return std::experimental::nullopt;
+  }
+
+  /**
+   * Adds an index for the given (label, property) and populates it with
+   * existing vertices that belong to it.
+   *
+   * You should never call BuildIndex on a GraphDbAccessor (transaction) on
+   * which new vertices have been inserted or existing ones updated. Do it in a
+   * new accessor instead.
+   *
+   * Build index throws if an index for the given (label, property) already
+   * exists (even if it's being built by a concurrent transaction and is not yet
+   * ready for use).
    *
    * @param label - label to build for
    * @param property - property to build for

tests/unit/graph_db_accessor.cpp

@@ -1,3 +1,4 @@
+#include <experimental/optional>
 #include "gtest/gtest.h"
 
 #include "database/graph_db.hpp"
@@ -325,6 +326,32 @@ TEST(GraphDbAccessorTest, Properties) {
   EXPECT_NE(prop, dbms.active()->property("surname"));
 }
 
+TEST(GraphDbAccessorTest, Transfer) {
+  Dbms dbms;
+
+  auto dba1 = dbms.active();
+  auto prop = dba1->property("property");
+  VertexAccessor v1 = dba1->insert_vertex();
+  v1.PropsSet(prop, 1);
+  VertexAccessor v2 = dba1->insert_vertex();
+  v2.PropsSet(prop, 2);
+  EdgeAccessor e12 = dba1->insert_edge(v1, v2, dba1->edge_type("et"));
+  e12.PropsSet(prop, 12);
+
+  // make dba2 that has dba1 in it's snapshot, so data isn't visible
+  auto dba2 = dbms.active();
+  EXPECT_EQ(dba2->Transfer(v1), std::experimental::nullopt);
+  EXPECT_EQ(dba2->Transfer(e12), std::experimental::nullopt);
+
+  // make dba3 that does not have dba1 in it's snapshot
+  dba1->commit();
+  auto dba3 = dbms.active();
+  // we can transfer accessors even though the GraphDbAccessor they
+  // belong to is not alive anymore
+  EXPECT_EQ(dba3->Transfer(v1)->PropsAt(prop).Value<int64_t>(), 1);
+  EXPECT_EQ(dba3->Transfer(e12)->PropsAt(prop).Value<int64_t>(), 12);
+}
+
 int main(int argc, char **argv) {
   ::testing::InitGoogleTest(&argc, argv);
   //  ::testing::GTEST_FLAG(filter) = "*.DetachRemoveVertex";