Scala Play JSON trouble converting List[CaseClass] to Json String - scala

Question

Not sure where I am going wrong with this it is returning error:
No Json serializer as JsObject found for type List[QM_Category].
Try to implement an implicit OWrites or OFormat for this type.
[error] Json.stringify(Json.toJsObject(a.categories))
Is there a way to define a format for just List[QM_Category]? I thought the format for QM_Category would handle the case class and play is supposed to handle Lists...
All I really want to do is take my List and convert it to json string. Pretty straight forward but I am not sure why Play Json doesnt like my format.
Here is my code:
case class QM_Answer (
answerid: String,
answerstring: String,
answerscore: Int
);
case class QM_Question (
questionid: String,
questionscore: Int,
questiongoal: Int,
questionstring: String,
questiontype: String,
questioncomments: String,
questionisna: Boolean,
questionishidden: Boolean,
failcategory: Boolean,
failform: Boolean,
answers: List[QM_Answer]
);
case class QM_Category (
categoryid: String,
categoryname: String,
categoryscore: Int,
categorygoal: Int,
categorycomments: String,
categoryishidden: Boolean,
failcategory: Boolean,
questions: List[QM_Question]
);
case class SurveySourceRaw (
ownerid: String,
formid: String,
formname: String,
sessionid: String,
evaluator: String,
userid: String,
timelinekey: Long,
surveyid: String,
submitteddate: Long,
month: String,
channel: String,
categories: List[QM_Category]
);
case class SurveySource (
ownerid: String,
formid: String,
formname: String,
sessionid: String,
evaluator: String,
userid: String,
timelinekey: Long,
surveyid: String,
submitteddate: Long,
month: String,
channel: String,
categories: String
);
implicit val qmAnswerFormat = Json.format[QM_Answer];
implicit val qmQuestionFormat = Json.format[QM_Question];
implicit val qmCategoryFormat = Json.format[QM_Category];
implicit val surveySourceRawFormat = Json.format[SurveySourceRaw];
var surveySourceRaw = sc
.cassandraTable[SurveySourceRaw]("mykeyspace", "mytablename")
.select("ownerid",
"formid",
"formname",
"sessionid",
"evaluator",
"userid",
"timelinekey",
"surveyid",
"submitteddate",
"month",
"channel",
"categories")
var surveyRelational = surveySourceRaw
.map(a => SurveySource
(
a.ownerid,
a.formid,
a.formname,
a.sessionid,
a.evaluator,
a.userid,
a.timelinekey,
a.surveyid,
a.submitteddate,
a.month,
a.channel,
Json.stringify(Json.toJsObject(a.categories))
))

Answer 1

The Play JSON format for a List[A], given a format for A, encodes/decodes a JSON array, e.g. for a List[String] [ "foo", "bar", "baz" ]. A JSON array is not a JSON object.
So if you want the List[QM_Category] to be a stringified JSON (but not necessarily a JSON object, e.g. it could be a string, array, etc.), you can use toJson:
Json.stringify(Json.toJson(a.categories))
Alternatively, if you want it to be a JSON object, you would need to define an OFormat (or an OReads/OWrites combination) for List[QM_Category]: an OFormat is a Format which requires that the JSON be an object with string attributes and JSON values (and so forth for OReads/OWrites).

Answer 2

I'm almost embarresed to answer this but sometimes I make it overly complicated.
The answer was to just read the column from cassandra as a string instead of a List[QM_Category]. The column in cassandra was defined as:
categories list<FROZEN<qm.category>>,
I wrongfully assumed I would need to read it in from Cassandra as a list of custom objects. I would then need to use play json to format that class into JSON and then stringify it.
case class SurveySourceRaw (
ownerid: String,
formid: String,
formname: String,
sessionid: String,
evaluator: String,
userid: String,
timelinekey: Long,
surveyid: String,
submitteddate: Long,
month: String,
channel: String,
categories: List[QM_Category]
);
When in reality, all I needed to do was read it from cassandra as a type String and it came in as a stringified json. Well played spark cassandra connector, well played.
case class SurveySourceRaw (
ownerid: String,
formid: String,
formname: String,
sessionid: String,
evaluator: String,
userid: String,
timelinekey: Long,
surveyid: String,
submitteddate: Long,
month: String,
channel: String,
categories: String
);