Kotlin Android Room handle empty list of objects in Moshi TypeConverter-CodePudding

I inherited an Android project where I had to rebase the rxJava code to a more pure Kotlin centric code base with a MVVM design pattern. Given my inexperience, along with lots of head scratching and reading, I finally finished the code conversion. Sigh!
But, although testing has gone quite smooth, the new @TypeConverters seems more strict and will not allow empty lists of expected objects to store in the Room database.
When the data loads from the server and the item soil_types is empty e.g. soil_types=[], I get the error: Expected receiver of type models.SoilType, but got java.util.ArrayList.
Here is the converter:

@TypeConverter
fun fromJson(json: String?): List<SoilType>? {
    if(json == null) return null
    return convertJsonToListObject(json)
}

@TypeConverter
fun toJson(objectData: List<SoilType>?): String? {
    if (objectData == null) return null
    return convertListObjectToJson(objectData)
}
//...
inline fun <reified T> convertJsonToListObject(json: String): List<T> =
    moshi.adapter<List<T>>(T::class.java).fromJson(json).orEmpty()

inline fun <reified T> convertListObjectToJson(objectData: List<T>): String =
    moshi.adapter<List<T>>(T::class.java).toJson(objectData)

Here is the Room model to be populated:

@Entity(tableName = "soil")
data class Soil(
        @PrimaryKey val id: Long,
        val user_id: Long,
        val name: String,
        val soil_types: List<SoilType>,
        val updated_at: String,
        val created_at: String
)

I know this is not a good design of a data table column and the relationships that could be built.

My question is how can I allow the converter to accept the empty ArrayList, so I can save the emptyList as String resembling '{}'? Or, I do not want to do this, should I allow for null?

Edit:
I tried changing the converter to the following:

@TypeConverter
fun fromJson(json: String?): List<SoilType> {
    if(json == null || json == "{}") return emptyList()
    return convertJsonToListObject(json)
}

@TypeConverter
fun toJson(objectData: List<SoilType>?): String {
    if (objectData.isNullOrEmpty()) return "{}"
    return convertListObjectToJson(objectData)
}

But the error remains.

Edit 2:
It seems what I am experiencing is built into the Moshi json library . Upon further reading in section Moshi handling of null and absent JSON fields; it seems default values must be declared for data class fields to get expected results:

If the JSON response changes and sets a null field in the JSON then the adapter will fail respecting the non null reference of a val property in the data class and throw a clear exception.

In this case a List of objects that is empty.

CodePudding user response：

The problem is actually not in Room but in Moshi. Based on its documentation, you have to do the following in order to parse JSON lists:

inline fun <reified T> convertJsonToListObject(json: String): List<T> =
    moshi.adapter<List<T>>(Types.newParameterizedType(List::class.java, T::class.java)).fromJson(json).orEmpty()

inline fun <reified T> convertListObjectToJson(objectData: List<T>): String =
    moshi.adapter<List<T>>(Types.newParameterizedType(List::class.java, T::class.java)).toJson(objectData)

There is also a Kotlin-specific way of doing this, but in Moshi's latest release (1.13.0), you still need to opt in to this feature:

import com.squareup.moshi.adapter

@OptIn(ExperimentalStdlibApi::class)
inline fun <reified T> convertJsonToListObject(json: String): List<T> =
    moshi.adapter<List<T>>().fromJson(json).orEmpty()

@OptIn(ExperimentalStdlibApi::class)
inline fun <reified T> convertListObjectToJson(objectData: List<T>): String =
    moshi.adapter<List<T>>().toJson(objectData)

Arguably, this only complicates your code so it may be worth waiting before using this Kotlin-specific solution.