Why am I getting this "bad value for restricted picklist field" in my test class?

Question

When trying to assign values to a multi-picklist field in test class, I'm facing the above error. Please take a look below my controller and test class.

Controller

    List<String> issues = (List<String>) JSON.deserialize(caseDetail.Test__c, List<String>.class);
    for(String strIssue : issues) {
        if(delimiter == '' || delimiter == null) {
            delimiter = strIssue;
        }
        else {
            delimiter += ';' + strIssue;
        }
    }
    NewCaseToUpdate.Test__c = delimiter;

Test class :

    List<String> issueList = new List<String>();
    issueList.add('Engineering Samples');
    testCase.Test__c = JSON.serialize(issueList);

Answer 1

You don't need to serialize a string list into a JSON list to store the values in your field. Multi-picklist fields use the ; character to separate the selected values.

You are trying to assign a list like this:

["Engineering Samples"]

While your field likely expects just a single value like this:

"Engineering Samples"

Or, if you want to test multiple values, you should assign them like this:

"Engineering Samples"; Others; "Butterfly Samples"

Notice: they are separated by ;. If a value does not contain a space, and if it isn't even surrounded by double quotes.

From the documentation:

Multi-select picklist fields contain a list of one or more items from which a user can choose multiple items. One of the items can be configured as the default item. Selections are maintained as a string containing a series of attributes delimited by semicolons. For example, a query can return the values of a multivalue picklist as “first value; second value; third value”. For information on querying multi-select picklists, see Querying Multi-Select Picklists in the Salesforce SOQL and SOSL Reference Guide.

More here: Querying Multi-Select Picklists.