diff --git a/CHANGELOG.md b/CHANGELOG.md
index fbb24f6..6c46179 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -3,6 +3,7 @@
 ## Not released
 
 - Add `.to_json` and `.as_json` to `SObject` to allow JSON serialization (https://github.com/Beyond-Finance/active_force/pull/37)
+- Memoize the `ActiveForce::Mapping#mappings` Hash since it is based on the fields and those are generally only set when the class is loaded. Also use `Hash#key` which returns the key for a value rather than `Hash#invert` which creates a new Hash with key/value inverted. (https://github.com/Beyond-Finance/active_force/pull/41)
 
 ## 0.13.0
 
diff --git a/lib/active_force/mapping.rb b/lib/active_force/mapping.rb
index c6f10b4..32e0827 100644
--- a/lib/active_force/mapping.rb
+++ b/lib/active_force/mapping.rb
@@ -13,7 +13,7 @@ def initialize model
     end
 
     def mappings
-      Hash[fields.map { |field, attr| [field, attr.sfdc_name] }]
+      @mappings ||= Hash[fields.map { |field, attr| [field, attr.sfdc_name] }]
     end
 
     def sfdc_names
diff --git a/lib/active_force/sobject.rb b/lib/active_force/sobject.rb
index a6511e7..5253dbf 100644
--- a/lib/active_force/sobject.rb
+++ b/lib/active_force/sobject.rb
@@ -180,7 +180,7 @@ def write_value key, value
         field = key
       else
         # Assume key is an SFDC column
-        field = mappings.invert[key]
+        field = mappings.key(key)
       end
       send "#{field}=", value if field && respond_to?(field)
     end