title: Ambiguous Type Error When Using RawSql Update
body: <p>I am trying to update a column of a table based on other columns but looks like I cannot do it with the updateWhere function. So I tried to use rawSQl but it does not work due to ambiguous type error. </p>
  
  <pre><code>getCalculateDeltaR :: personId -&gt; Handler Html
  getCalculateDeltaR personId = do
  
      goods &lt;- runDB $ selectList [GoodPerson ==. personId] [] 
  
      forM goods $ \(Entity gid good) -&gt; do
           let aid = goodAsset good  
           asset &lt;- runDB $ get404 aid
           let mktValue = assetMktValue asset
           runDB $ rawSql "UPDATE good SET  delta = (? - orig_value) \
                WHERE person = ? AND asset = ?" 
                [toPersistValue mktValue, toPersistValue personId, toPersistValue aid]
  
      defaultLayout $ do
          $(widgetFile "calculateDelta")
  </code></pre>
  
  <p>I get the following error and I did try adding ?? after UPDATE but it did not make a difference. </p>
  
  <pre><code>Ambiguous type variable `a0' in the constraint:
    (RawSql a0) arising from a use of `rawSql'
  Probable fix: add a type signature that fixes these type variable(s)
  In the second argument of `($)', namely
    `rawSql
   ...
   ...
  </code></pre>
  
  <p>The parameters that I can passing to rawSQL are well defined so not sure what's causing this issue. Could someone tell me how I should resolve the above issue? If there is a better way to update columns based on other columns, would love to know that as well. </p>
  
  <p>Thanks!</p>