Determining the different options of a pull-down menu from a database entity?

Question

I am trying to populate the values of a pull-down menu from a database. For this purpose, I defined cons!GENERIC_TASKS which points to corresponding database entity. I also have the following expression rule which reads all necessary data from the respective entity: 
 a!queryEntity( entity: cons!GENERIC_TASKS, query: a!query( selection: a!querySelection( columns: { a!queryColumn( field: "genericTaskName" ) } ), pagingInfo: a!pagingInfo( startIndex: 1, batchSize: -1 ) ), fetchTotalCount: false ) 
 The test result of this rule looks promising since it contains all required data, the labels and the values. 
 
 In my user interface, I am now trying the following: 
 a!dropdownField( label: "Generic step type", labelPosition: "ABOVE", placeholder: "--- Please choose a step type ---", choiceLabels: {rule!APAS_GetAllGenericTaskNames.data.genericTaskName}, choiceValues: {rule!APAS_GetAllGenericTaskNames.data.identifiers}, saveInto: {}, searchDisplay: "AUTO", validations: {} ) 
 However, this gives me the following error: 
 Could not display interface. Please check definition and inputs. Interface Definition: Expression evaluation error at function a!dropdownField: Invalid index: Cannot index property 'data' of type Text into type Rule or Function Reference 
 What do I do wrong?

Arun Dubey · Accepted Answer

choiceValues: {rule!APAS_GetAllGenericTaskNames.data.identifiers} Why are you using identifiers ? I don't know your use case but if you are trying to store or display then use same for both choiceLabels and choiceValues. 
 choiceLabels: {rule!APAS_GetAllGenericTaskNames().data.genericTaskName}, choiceValues: {rule!APAS_GetAllGenericTaskNames().data. genericTaskName },

Mike Schmitt · Answer

Additionally, the way you're trying to refer to "identifiers" won't work. "Identifiers" is not a sub-property of ".data", but instead a property of the query result (i.e. the data subset returned by a!queryEntity). 
 I suggest that instead of trying to mess with "identifiers", just include the primary key field in the list of columns you're including in your a!querySelection(); then you can refer directly to the primary key field by its field name instead of trying to use ".identifiers".

Mike Schmitt · Answer

Why are you running the same query twice, once for choiceLabels and once for choiceValues? That seems pretty inefficient. 
 Instead you should be doing something like this, querying just once into a local variable and then referring to that subsequently. a!localVariables(
 local!taskNames: rule!APAS_GetAllGenericTaskNames().data,
 
 a!dropdownField(
 label: "Generic step type",
 labelPosition: "ABOVE",
 placeholder: "--- Please choose a step type ---",
 choiceLabels: local!taskNames.genericTaskName,
 choiceValues: local!taskNames.identifiers,
 saveInto: {},
 searchDisplay: "AUTO",
 validations: {}
 )
) 
 As an aside: you've posted code a few different ways above, but I universally recommend using the "Code Box" i've used here. For reference:

Determining the different options of a pull-down menu from a database entity?

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