How to use Spark graph's function mask?
<p>I want to check out if a new graph(called A) is the sub-graph of other graph(called B). And i write a little demo for test, but failed! I run the demo just on spark-shell, spark version 1.6.1:</p>

<pre><code>// Build the GraphB
val usersB = sc.parallelize(Array(
  (3L, ("rxin", "student")),
  (7L, ("jgonzal","postdoc")),
  (5L, ("franklin", "prof")),
  (2L, ("istoica", "prof"))
))

val relationshipsB = sc.parallelize(Array(
  Edge(3L, 7L, "collab"),
  Edge(5L, 3L, "advisor"),
  Edge(2L, 5L, "colleague"),
  Edge(5L, 7L, "pi")
))

val defaultUser = ("John Doe", "Missing")

val graphB = Graph(usersB, relationshipsB, defaultUser)

// Build the initial Graph A
val usersA = sc.parallelize(Array(
  (3L, ("rxin", "student")),
  (7L, ("jgonzal", "postdoc")),
  (5L, ("franklin", "prof"))
))

val relationshipsA = sc.parallelize(Array(
  Edge(3L, 7L, "collab"),
  Edge(5L, 3L, "advisor")
))

val testGraphA = Graph(usersA, relationshipsA, defaultUser)

//do the mask
val maskResult = testGraphA.mask(graphB)
maskResult.edges.count
maskResult.vertices.count
</code></pre>

<p>In my understanding of <a href="http://spark.apache.org/docs/latest/api/scala/index.html#org.apache.spark.graphx.Graph" rel="nofollow">API on spark website</a>, mask funciton could get all the same edges and vertices. However, the result is vertices is correct only(    maskResult.vertices.count = 3), the count of edges should be 2 but not(maskResult.edges.count = 0).</p>