diff --git a/gtsam/discrete/DiscreteKey.cpp b/gtsam/discrete/DiscreteKey.cpp
index 5ddad22b0..0857f8c1e 100644
--- a/gtsam/discrete/DiscreteKey.cpp
+++ b/gtsam/discrete/DiscreteKey.cpp
@@ -38,11 +38,9 @@ namespace gtsam {
     return js;
   }
 
-  map<Key,size_t> DiscreteKeys::cardinalities() const {
-    map<Key,size_t> cs;
-    cs.insert(begin(),end());
-//    for(const DiscreteKey& key: *this)
-//      cs.insert(key);
+  map<Key, size_t> DiscreteKeys::cardinalities() const {
+    map<Key, size_t> cs;
+    cs.insert(begin(), end());
     return cs;
   }
 
diff --git a/gtsam/discrete/DiscreteKey.h b/gtsam/discrete/DiscreteKey.h
index ae4dac38f..ce0c56dbe 100644
--- a/gtsam/discrete/DiscreteKey.h
+++ b/gtsam/discrete/DiscreteKey.h
@@ -28,8 +28,8 @@
 namespace gtsam {
 
   /**
-   * Key type for discrete conditionals
-   * Includes name and cardinality
+   * Key type for discrete variables.
+   * Includes Key and cardinality.
    */
   using DiscreteKey = std::pair<Key,size_t>;
 
@@ -45,6 +45,11 @@ namespace gtsam {
     /// Construct from a key
     explicit DiscreteKeys(const DiscreteKey& key) { push_back(key); }
 
+    /// Construct from cardinalities.
+    explicit DiscreteKeys(std::map<Key, size_t> cardinalities) {
+      for (auto&& kv : cardinalities) emplace_back(kv);
+    }
+
     /// Construct from a vector of keys
     DiscreteKeys(const std::vector<DiscreteKey>& keys) :
       std::vector<DiscreteKey>(keys) {