In v2.0 of Web Services API collections are not hydrated automatically. Tasks on a User Story, Defects on a User Story, UserStories on a Feature, Iterations on a Project, etc. are examples of collections. For performance reasons v2.0 (unlike its predecessors - please see the API deprecation schedule here) does not allow getting the elements of the collection from the same request you made to get the parent object. You need a second request to load (hydrate) the collection.
However v2.0 has a concept of collection's summary. See WS API documentation for Collections.
EXAMPLE 1: Iterations collection on Project
Here is an example of an endpoint that fetches Iterations collection on Project without summarizing it:
https://rally1.rallydev.com/slm/webservice/v2.0/projects?fetch=Name,Iterations
Notice that even if fetch includes StartDate and EndDate, as in ?fetch=Name,Iterations,StarDate,EndDate those Iteration fields will not be included in the result. The iteration object returned by this endpoint is only a reference to the full object:
To summarize the collection by a desired fields, follow this syntax:
https://rally1.rallydev.com/slm/webservice/v2.0/projects?fetch=Name,Iterations:summary[StartDate;EndDate]
Notice Iterations:summary format: ?fetch=Name,Iterations:summary[StartDate;EndDate]
The iteration object returned by this endpoint includes StartDate and EndDate fields.
EXAMPLE 2: TestCases collection on HierarchicalRequirement
Here is an example of an endpoint that fetches TestCases collection on User Stories without summarizing it. User Stories are filtered by iteration:
https://rally1.rallydev.com/slm/webservice/v2.0/hierarchicalrequirement?workspace=https://rally1.rallydev.com/slm/webservice/v2.0/workspace/12352608129&query=(Iteration = /iteration/40695561076)&fetch=TestCases&pagesize=200
The same endpoint where TestCases are summarized by LastVerdict attribute of TestCase object:
https://rally1.rallydev.com/slm/webservice/v2.0/hierarchicalrequirement?workspace=https://rally1.rallydev.com/slm/webservice/v2.0/workspace/12352608129&query=(Iteration = /iteration/40695561076)&fetch=TestCases:summary[LastVerdict]&pagesize=200
Here one of the stories scheduled for this iteration, a story "abc" has 3 test cases in its TestCases collection. Two of them have LastVerdict "Pass". The third test case does not have a verdict, hence "No Entry".
The query above can be modified to include a summary of TestCases by FormattedID in addition to LastVerdict
https://rally1.rallydev.com/slm/webservice/v2.0/hierarchicalrequirement?workspace=https://rally1.rallydev.com/slm/webservice/v2.0/workspace/12352608129&query=(Iteration = /iteration/40695561076)&fetch=TestCases:summary[FormattedID;LastVerdict]&pagesize=200