Thanks for the idea. I forgot that I can use ordinals of fields.
Changing ```flatMapTo('BidRequest -> 'outField)``` to ```flatMapTo(0 -> 'outField)``` makes it possible to use JobTest with PackedAvroSource.

Unfortunately mocking PackedAvroSource in JobTest doesn't work when I use field name as ```'field```. Exception is thrown:

```
[info] CollectSitesJobTest:
[info] - collect URLs *** FAILED ***
[info]   cascading.flow.planner.PlannerException: could not build flow from assembly: [[com.twitter.scalding.a...][com.twitter.scalding.RichPipe.eachTo(RichPipe.scala:494)] unable to resolve argument selector: [{1}:'BidRequest'], with incoming: [{1}:0]]
[info]   at cascading.flow.planner.FlowPlanner.handleExceptionDuringPlanning(FlowPlanner.java:576)
[info]   at cascading.flow.hadoop.planner.HadoopPlanner.buildFlow(HadoopPlanner.java:265)
[info]   at cascading.flow.hadoop.planner.HadoopPlanner.buildFlow(HadoopPlanner.java:80)
[info]   at cascading.flow.FlowConnector.connect(FlowConnector.java:459)
[info]   at com.twitter.scalding.Job.buildFlow(Job.scala:226)
[info]   at com.twitter.scalding.Job.run(Job.scala:275)
[info]   at com.twitter.scalding.JobTest.runJob(JobTest.scala:172)
[info]   at com.twitter.scalding.JobTest.runHadoop(JobTest.scala:135)
[info]   at similarityScoreBid.CollectSitesJobTest$$anonfun$1.apply$mcV$sp(CollectSitesJobTest.scala:25)
[info]   at similarityScoreBid.CollectSitesJobTest$$anonfun$1.apply(CollectSitesJobTest.scala:13)
[info]   ...
[info]   Cause: cascading.pipe.OperatorException: [com.twitter.scalding.a...][com.twitter.scalding.RichPipe.eachTo(RichPipe.scala:494)] unable to resolve argument selector: [{1}:'BidRequest'], with incoming: [{1}:0]
[info]   at cascading.pipe.Operator.resolveArgumentSelector(Operator.java:345)
[info]   at cascading.pipe.Each.outgoingScopeFor(Each.java:368)
[info]   at cascading.flow.planner.ElementGraph.resolveFields(ElementGraph.java:628)
[info]   at cascading.flow.planner.ElementGraph.resolveFields(ElementGraph.java:610)
[info]   at cascading.flow.hadoop.planner.HadoopPlanner.buildFlow(HadoopPlanner.java:250)
[info]   at cascading.flow.hadoop.planner.HadoopPlanner.buildFlow(HadoopPlanner.java:80)
[info]   at cascading.flow.FlowConnector.connect(FlowConnector.java:459)
[info]   at com.twitter.scalding.Job.buildFlow(Job.scala:226)
[info]   at com.twitter.scalding.Job.run(Job.scala:275)
[info]   at com.twitter.scalding.JobTest.runJob(JobTest.scala:172)
[info]   ...
[info]   Cause: cascading.tuple.FieldsResolverException: could not select fields: [{1}:'BidRequest'], from: [{1}:0]
[info]   at cascading.tuple.Fields.indexOf(Fields.java:1008)
[info]   at cascading.tuple.Fields.select(Fields.java:1064)
[info]   at cascading.pipe.Operator.resolveArgumentSelector(Operator.java:341)
[info]   at cascading.pipe.Each.outgoingScopeFor(Each.java:368)
[info]   at cascading.flow.planner.ElementGraph.resolveFields(ElementGraph.java:628)
[info]   at cascading.flow.planner.ElementGraph.resolveFields(ElementGraph.java:610)
[info]   at cascading.flow.hadoop.planner.HadoopPlanner.buildFlow(HadoopPlanner.java:250)
[info]   at cascading.flow.hadoop.planner.HadoopPlanner.buildFlow(HadoopPlanner.java:80)
[info]   at cascading.flow.FlowConnector.connect(FlowConnector.java:459)
[info]   at com.twitter.scalding.Job.buildFlow(Job.scala:226)
[info]   ...
```