drools - Matching a list (of tags) with another and detecting presence of common elements

Question

My requirement is to match tags. In the example, this particular HourConstraint checks the TeacherHour assigned to Hour(23).

Specifically, it checks TeacherHour.attributes["tags"] for the values ["asst_ct","teacher_john_smith"] and detects atleast one match, two in this case (both "asst_ct" and "teacher_john_smith") .

TeacherHour:
  id: 47
  assigned_hour: Null
  attributes:Map<List<String>>
    "tags":["asst_ct","no_strenuous_duties","kinda_boring","teacher_john_smith"]
    "another_attribute":[...]

HourConstraint:
   hour: Hour(23)
   attribute: "tags"
   values_list: ["asst_ct","teacher_john_smith"]

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Question: How do I detect the presence (true or false) of common elements between two lists?

Drools Expert has memberOf and contains, but they check a scalar vs a collection, never a collection vs a collection.

I see two potential ways:

1. introduce a function boolean isIntersecting(list,list) and tell Drools to use that for truth checking
2. Implement TeacherHour.attributes[] as a string instead of a list and HourConstraint.valueslist as a regular expression that can match that list

Answer 1

There are a few options. Most straight forward is to use the Collections class to do that for you:

rule X
when
    $t: TeacherHour( )
    HourConstraint( Collections.disjoint( $t.attributes["tags"], values_list ) == false )
...

If this is something you would use often in your rules, then I recommend wrapping that function in a pluggable operator, supported by Drools. Lets say you name the operator "intersect", you can then write your rules like this:

rule X
when
    $t: TeacherHour( )
    HourConstraint( values_list intersect $t.attributes["tags"] )
...

A third option, is to use "from", but that is less efficient in runtime as it causes iterations on the first list:

rule X
when
    $t: TeacherHour( )
    $tag : String() from $t.attributes["tags"]
    exists( HourConstraint( values_list contains $tag ) )
...