unable to use if else in Expression rule

Question

a!localVariables(
 
 local!reassigndetail: a!queryEntity(
 entity: cons!MPPCONSDSE_MPPTABSAM_task,
 query: a!query(
 selection: a!querySelection(
 columns: {
 a!queryColumn(
 field: "ReassignEmail"
 ),
 a!queryColumn(
 field: "samaccid"
 )
 }
 ),
 logicalExpression: a!queryLogicalExpression(
 operator: "AND",
 filters: {
 a!queryFilter(
 field: "Reassign",
 operator: "=",
 value: "Yes"
 ),
 a!queryFilter(
 field: "annoucncementid",
 operator: "=",
 value: ri!Announcementid
 )
 },
 ignoreFiltersWithEmptyValues: true
 ),
 pagingInfo: a!pagingInfo(
 startIndex: 1,
 batchSize: -1
 )
 ),
 fetchTotalCount: false
 ).data,

 
 local!fabdetails:
 a!forEach(
 items:local!reassigndetail,
 expression:rule!MPPER_getFabsnamefromsamaccid(fv!item.samaccid)

 ),
 local!fabdetails2: a!forEach(a!flatten( local!fabdetails).fabname,fv!item),
 local!samemails: a!forEach(a!flatten( local!reassigndetail).ReassignEmail,fv!item)
 ,

 
 local!samdata : a!queryEntity(
 entity: cons!MPPCONSCDT_SAMAccountability,
 query: a!query(
 logicalExpression: a!queryLogicalExpression(
 operator: "AND",
 filters: {
 a!queryFilter(
 field: "samemail",
 operator: "in",
 value:local!samemails
 ),
 a!queryFilter(
 field: "fabname",
 operator: "in",
 value:local!fabdetails2
 )
 },
 ignoreFiltersWithEmptyValues: true
 ),
 pagingInfo: a!pagingInfo(
 startIndex: 1,
 batchSize: 50
 )
 ),
 fetchTotalCount: false
 ).data,
 
 local!result:
 a!forEach(
 items:local!samdata,
 expression: cast(typeof('type!{urn:com:appian:types}MPPSAMAccoumtability'()),fv!item)
 ),
 local!result

 
 
)

this works fine if local!reassigndetail finds some data from DB using a!queryentity but when it doesn't return any data from db then it gives following error 
 
 Expression evaluation error at function a!forEach [line 47]: Invalid index: Cannot index property &rsquo;&rsquo;fabname&rsquo;&rsquo; of type String into type List of Variant 
 
 Please help

Dave Lewis · Answer

I see what you're doing. 
 You're doing a!forEach( 
 items: a!flatten(list).property, 
 expression: fv!item 
 ) 
 There's absolutely 0 point to using a!forEach. The flatten function already takes a list. It flattens a nested list into a single list, destroying the more complex structure. The expression only returns the items. Well, if a!flatten(list).property worked, you'd already have a list of results without even using a!forEach. 
 My advice, make a separate rule that only goes up to that point then returns nothing but a!flatten(your list) and see what that does. Then, see if you can actually use the dot notation to get the property you want out of it. Then, see if you need a!forEach. I don't think you will.

Mike Schmitt · Answer

List of what / what is the structure of the list returned? Because your subsequent code assumes that local!fabDetails is a dictionary containing a property named fabName .

Dave Lewis · Answer

When the database returns 0 rows, it takes the form of "{}", which has the type "List of Variant". You need to return at least one row with data in it before Appian can be aware of what the fields are even supposed to be. You don't even get the structure without returning at least a row of data. 
 So, the solution is to wrap your logic inside an if function, which contains three parameters: 
 The first parameter uses APN_isEmpty() or length() = 0 to determine if you returned 0 rows, 
 The second parameter returns whatever you want to return in the case of 0 rows of data being returned by the query, could be as simple as "{}", 
 The third parameter is what you were doing before, which only works when you have data to work on. 
 Over time, you'll learn to wrap practically everything you write in "if is empty" or "if is blank". A great many of us call it "null safety."

unable to use if else in Expression rule

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