Adding rule condition based on two objects in drools - drools

Question

> { "batch-execution":{
> "lookup":"defaultKieSession",
> "commands":[
> {
> "insert":{
> "out-identifier":"FieldData1",
> "object":{
> "FieldData":{
> "name":"abc",
> "value":"111"
> }
> }
> }
> },
> {
> "insert":{
> "out-identifier":"FieldData2",
> "object":{
> "FieldData":{
> "name":"xyz",
> "value":"222"
> }
> }
> }
> },
> {
> "fire-all-rules":{
>
> }
> }
> ] } }
Now i want to write a condition in drl similar to this:
rule "testrule"
when
fieldData(name == "abc" , value == "111") && fieldData(name == "xyz",value = "222")
then
System.out.println("Condition executed")
Can someone help on how this can be done in drools ?

Answer 1

Of course you can! Your "example" rule is nearly perfect as-is.
The way drool works is that it evaluates the conditions of all of the objects in working memory, and only if all conditions are met will it trigger that rule.
So you could write a rule that looks like this:
rule "Test Rule"
when
exists( FieldData( name == "abc", value == "111") )
exists( FieldData( name == "xyz", value == "222") )
then
System.out.println("Condition Executed")
end
This rule will trigger if there exists an object in working memory that has a name of 'abc' and a value of '111', and there also exists an object in working memory with the name of 'xyz' and value of '222'.
In the example above, I used the 'exists' predicate because we weren't going to actually be doing anything with those values, and we just wanted to confirm that there is such a FieldData object in memory that matches the required conditions.
Note that this assumes that you've entered your FieldData objects directly into the working memory as standalone objects. (Eg you fired the rules and passed it a List of FieldData.) If you're working with bigger structures, you'll have to extract the FieldData objects, and then do an exists check like I had in my previous example.
For example, let's say you had a set of classes like this (which mimic your example JSON; getters and setters omitted for brevity):
class FieldData {
String name;
String value;
}
class Command {
String outIdentified;
FieldData object;
}
class BatchExecution {
String action; // eg "insert"
String lookup;
List<Command> commands;
}
If you passed a BatchExecution into the rules, you'll need to pull the field data out of the commands before you can check that two FieldData exist with the conditions you want. Your rule would therefore look more like this:
rule "Test Rule 2"
when
// Get the BatchExecution in working memory and pull out the Commands
BatchExecution( $commands: commands != null )
// Get the FieldData from each of the commands
$data: ArrayList( size >= 2) from
accumulate( Command( $fd: object != null ) from $commands,
init( ArrayList fieldDatas = new ArrayList() ),
action( fieldDatas.add( $fd ) ),
reverse( fieldDatas.remove( $fd ) ),
result( fieldDatas ))
// Confirm there exist FieldData with our criteria inside of the Commands
exists( FieldData( name == "abc", value == "111" ) from $data
exists( FieldData( name == "xyz", value == "222" ) from $data
then
System.out.println("Condition executed, 2");
end
Basically what we have to do is drill down from the object actually inserted into working memory until we can get to the objects that we need to be doing work against, in this case the FieldData. I used the accumulate aggregate to pull all of the FieldData into a List that we then check for the presence of the two FieldData that we're looking for in this particular rule.
For more information, you should consider reading the Drools documentation, specifically the part on the "Rule Language Reference" (section 4) which is very well written and contains plenty of examples that you can adapt or expand upon.

Answer 2

The rule firing in drools only happens on the occurrence of some event. Read about sessions and execution of rules here.
Coming to the your question of writing the above rule. I am not sure how you want the rule to be executed. As per my understanding if you want to write a separate rule each to check name as xyz and abc, then you can write the rule as below:
rule "testrule1"
when
fieldData(name == "abc" , value == "111")
then
System.out.println("Condition executed 1")
end
rule "testrule2"
when
fieldData(name == "xyz" , value == "222")
then
System.out.println("Condition executed2")
end
If you want to combine the rule then you can write it as:
rule "testrule"
when
fieldData(name == "abc" || name == "xyz" && value == "111" || value == "222")
then
System.out.println("Condition executed")
end
Note: You cannot define a rule like above if you want to work on the occurrence of multiple events. If you want to work on multiple events you can read about windowing in drools. Add more information on your use case to get better answers.