COLLECT FROM NAMED [WHERE]


Version of the COLLECT FROM [WHERE] function for complex nested selections with an alias.


Syntax

 COLLECT entity | attribute FROM collection NAMED alias [ WHERE ( expression ) ]


Inputs
  • entity or attribute - Entity or attribute to collect. This should contain the alias, e.g. alias.Name.
  • collection - A collection of entity instances.
  • alias - A name for the collection.
  • expression - Boolean expression that represents the criterion the instance has to meet.


Return type

  • collection of entity instances
  • collection of attribute values

A collections contains no duplicates. Intermediary COLLECT expressions can contain duplicates however, please see the note on collections and duplicates.


Examples

Suppose the following model with only instances of entity Person. Person.hasChildren is a relation from Person to Person.


Person instancePerson.NamePerson.AgePerson.hasChildren
Person_1“Kim”16
Person_2“Rick”38Person_3, Person_5
Person_3“Bob”8
Person_4“Julia”42Person_1
Person_5“Sam”3
Person_6“Joan”34Person_3, Person_5


To retrieve all the parent names, we use this expression:

COLLECT Parent.name 
FROM
  COLLECT Person 
  FROM ALL Person 
  WHERE ( Person.hasChildren != ? ) NAMED Parent


This expression results in “Rick”, “Julia” and “Joan”.

To retrieve all children names, we use this expression:

COLLECT Child
FROM (
  COLLECT Person.hasChildren 
  FROM ALL Person 
  WHERE ( Person.hasChildren != ? ) ) NAMED Child


This expression results in a collection of three instances of Person (i.e Person_1, Person_3 and Person_5, with names Kim, Bob and Sam).

To retrieve all children younger than 15, we use this expression:

COLLECT Child 
FROM (
  COLLECT Person.hasChildren
  FROM ALL Person 
  WHERE ( Person.hasChildren != ? ) ) NAMED Child
WHERE ( Child.Age < 15 )


This expression results in a collection of two instances of Person (i.e Person_3 and Person_5, with names Bob and Sam).

To retrieve the children’s names with a parent older than 40 years:

COLLECT Child.Name 
FROM (
  COLLECT Person.hasChildren 
  FROM ALL Person 
  WHERE ( Person.hasChildren != ? AND Person.Age > 40 ) ) NAMED Child


This expression results in a collection with only the name “Kim”.

You can make it as complex as you like, for example:

COLLECT Child.Name 
FROM (
  COLLECT Parent.hasChildren
  FROM (
    COLLECT Person
    FROM ALL Person 
    WHERE ( Person.hasChildren != ? ) ) NAMED Parent
  WHERE ( Parent.Age > 40 )
) NAMED Child 
WHERE ( Child.Age < 18 )


This expression results also in a collection with only the name “Kim”.

In some occasions, a local variable could be a good alternative for using an alias. E.g. if you want a collection of all persons except the person that is currently active, this would be simple and transparent:

thisPerson := Person ;
 
COLLECT Person
    FROM ALL Person 
    WHERE ( Person != thisPerson )