View

A view in HiLang represents a summary/subset of one or more entities where the projection is defined by the source entity. The most useful views are Node and Edge that enable any model to be viewed as nodes.

Views can be created in two ways

1. entity Cousins.Person= Node () where every member of the view has a corresponding member within the entity
2. entity Cousins.Person= Edge (From = this, To = Mother, Name = Name, TypeName = "Mother") where expressions

The Plan demonstrates the definition of view.

entity Plan.Project : Versioned
                    = Node (SKey = SKey, Name = Name, TypeName = "Plan"),
                      BI.Item (WBS         = WBS, 
                               Name        = Name,
                               ActualCost  = ActualCost, 
                               PlanCost    = PlanCost, 
                               ImpliedCost = ImpliedCost,
                               Plan        = Duration, 
                               Actual      = Duration,
                               TypeName    = "Plan")

entity Plan.Tasks.Task : Versioned
                       = Node (SKey = SKey, Name = Name, TypeName = "Plan-Task"),
                         Edge (From = Project, To = this, TypeName = "Plan-Task", Name = Name),
                         Plan.Tasks.SubTask (Task = this, Parent = Parent),
                         BI.Item (WBS         = BIWBS, 
                                  Name        = Name,
                                  ActualCost  = ActualCost, 
                                  PlanCost    = PlanCost, 
                                  ImpliedCost = ImpliedCost,
                                  Plan        = Duration, 
                                  Actual      = Actual.Duration,
                                  TypeName    = Parent = null ? "Task" : "SubTask")

segment Plan.Tasks.TaskResource  = Node (SKey = SKey, Name = Resource.Name, TypeName = "Plan-Task-Resource"), 
                                   Edge (From = owner, To = Resource, Name = Resource.Name, TypeName = "Plan-Task-Resource")

entity Plan.Resource : Versioned
                     = Node (SKey = SKey, Name = Name, TypeName = "Plan-Resource")

view BI.Item    = Node (SKey = SKey, Name = Name, TypeName = "BI-Cost-" + TypeName)
(
    WBS         : String,
    TypeName    : String
)
{
    Name        : String,
    ActualCost  : Decimal, 
    PlanCost    : Decimal, 
    ImpliedCost : Decimal,
    Plan        : Plan.Duration,
    Actual      : Plan.Duration
}
[
    PlanStart   = Plan.Start,
    PlanEnd     = Plan.End,
    PlanTime    = Plan.Time,
    ActualStart = Actual.Start,
    ActualEnd   = Actual.End,
    ActualTime  = Actual.Time
];

is equvilent to the SQL definition

CREATE OR REPLACE VIEW BI_ITEM 
    ( WBS,
      TypeName,
      Name,
      ActualCost,
      PlanCost,
      ImpliedCost,
      Plan,
      PlanStart,
      PlanEnd,
      PlanTime,
      ActualStart,
      ActualEnd,
      ActualTime
      ) AS
    SELECT  WBS, 
            'Plan'
            Name,
            ActualCost, 
            PlanCost, 
            ImpliedCost,
            Duration_Start, 
            Duration_End, 
            Duration_Time, 
            Duration_Start, 
            Duration_End, 
            Duration_Time, 
    FROM    Plan_Project 
    UNION ALL 
    SELECT  WBS, 
            'Task'
            Name,
            ActualCost, 
            PlanCost, 
            ImpliedCost,
            Duration_Start, 
            Duration_End, 
            Duration_Time, 
            Actual.Start, 
            Actual.End, 
            Actual.Time, 
            Plan_Tasks_Task.Duration, 
            Actual.Duration,
    FROM    Plan_Tasks_Task LEFT OUTER JOIN
            Actual ON Plan_Tasks_Task.Id = Actual.owner
    WHERE   Parent_Id IS NULL 
    UNION ALL 
    SELECT  WBS, 
            'Sub-Task'
            Name,
            ActualCost, 
            PlanCost, 
            ImpliedCost,
            Duration_Start, 
            Duration_End, 
            Duration_Time, 
            Actual.Start, 
            Actual.End, 
            Actual.Time, 
            Plan_Tasks_Task.Duration, 
            Actual.Duration,
    FROM    Plan_Tasks_Task LEFT OUTER JOIN
            Actual ON Plan_Tasks_Task.Id = Actual.owner
    WHERE   Parent_Id IS NOT NULL 
    ;

CREATE OR REPLACE VIEW Node (SKey, Name, TypeName) AS
    SELECT SKey, Name, 'Plan' FROM Plan_Project
    UNION ALL 
    SELECT SKey, Name, 'Plan-Task' FROM Plan_Tasks_Task
    UNION ALL
    SELECT SKey, Name, 'Plan-Resource' FROM Plan_Resource
    UNION ALL
    SELECT SKey, Name, 'Plan-Task-Resource' FROM Plan_Tasks_TaskResource
    UNION ALL
    SELECT SKey, Name, 'BI-Item-' || BI_ITEM.TypeName FROM  BI_ITEM 
    ;

CREATE OR REPLACE VIEW Edge ("From", "To", TypeName, Name) AS
    SELECT Project.SKey, Plan_Tasks_Task.SKey, 'Plan-Task', Name 
    FROM Plan_Tasks_Task INNER JOIN 
         Plan_Project AS Project ON Plan_Tasks_Task.Project_Id = Project.Id
    WHERE Parent_Id IS NULL
    UNION ALL
    SELECT Parent.SKey, Plan_Tasks_Task.SKey, 'Plan-Task-Part', Name 
    FROM Plan_Tasks_Task INNER JOIN 
         Plan_Tasks_Task as Parent ON Plan_Tasks_Task.Parent_Id = Id
    UNION ALL
    SELECT Plan_Tasks_Task.SKey, Resource.SKey, 'Plan-Task-Resource', Name 
    FROM Plan_Tasks_Task INNER JOIN 
         Plan_Resource AS Resource ON Plan_Tasks_Task.Resource_Id = Resource.Id
    ;   

HiLang syntax is much clearer, because the mapping of an entity / segment / aspect to a view is part of the definition of the element and not the view.

Views are evaluated in parallel in the HiLang generated code and use any key references to select elements. Node and Edge use the derived property SKey, which is a text encoding of the elements key which is decoded to only query elements that are of the key type. This is especially useful when navigating from Edge to a Node which queries Node using the key