When setting DB column to allow NULL in Database the app complains about the Schema

Question

I am trying to use Room in Android Studio using Kotlin with a pre-packaged database. The database does not set NOT NULL. Using DB SQL Browser it shows that the column has these properties

"Reference" TEXT

There is no NOT NULL. All the other columns in the table do have NOT NULL set.

In the Entity that maps that table I have:

@Entity
data class Meaning (
    @PrimaryKey(autoGenerate = true) @ColumnInfo(name = "Id") val id: Int,
    @NonNull @ColumnInfo(name = "Contents") val contents: String,
    /*
     * It's OK for Reference to be Null
     */
    @ColumnInfo(name = "Reference") val reference: String,
    @NonNull @ColumnInfo(name = "SymbolId") val symbolId: Int,
    @NonNull @ColumnInfo(name = "Local") val local: Int
    )

It builds and installs, but fails when running with this error:

java.lang.IllegalStateException: Pre-packaged database has an invalid schema: Meaning(<stuff>.Meaning).
     Expected:
TableInfo{name='Meaning', <unimportant columns>,  Reference=Column{name='Reference', type='TEXT', affinity='2', notNull=true, primaryKeyPosition=0, defaultValue='null'},<more unimportant columns>}
     Found:
TableInfo{name='Meaning', <unimportant columns>, Reference=Column{name='Reference', type='TEXT', affinity='2', notNull=false, primaryKeyPosition=0, defaultValue='null'}, <more unimportant columns}

Note that the Found has notNull=false, which seems correct because NOT NULL is not specified in the database. The Expected has notNull=true even though in the Entity, @NonNull was not specified for the Reference column.

So, I am confused why the Entity is expecting Reference column to be notNull=true.

Any pointers are welcome.

Answer 1

@ColumnInfo(name = "Reference") val reference: String?,

would equate to notNull = false.

i.e. the ? indicates null allowed.

So, I am confused why the Entity is expecting Reference column to be notNull=true.

Because the annotation processing sees String which cannot be null. Only if it sees String? then can the value be nullable.

That is if you you use:-

@Entity
data class Meaning (
    @PrimaryKey(autoGenerate = true) @ColumnInfo(name = "Id") val id: Int,
    @NonNull @ColumnInfo(name = "Contents") val contents: String,
    /*
     * It's OK for Reference to be Null
     */
    @ColumnInfo(name = "Reference") val reference: String?,
    @NonNull @ColumnInfo(name = "SymbolId") val symbolId: Int,
    @NonNull @ColumnInfo(name = "Local") val local: Int
)

and then compile, the generated Java (expected) for the @Database annotated class suffixed with _Impl includes:-

_db.execSQL("CREATE TABLE IF NOT EXISTS `Meaning` (`Id` INTEGER PRIMARY KEY AUTOINCREMENT NOT NULL, `Contents` TEXT NOT NULL, `Reference` TEXT, `SymbolId` INTEGER NOT NULL, `Local` INTEGER NOT NULL)");

i.e.

, `Reference` TEXT,

However without the ? then :-

_db.execSQL("CREATE TABLE IF NOT EXISTS `Meaning` (`Id` INTEGER PRIMARY KEY AUTOINCREMENT NOT NULL, `Contents` TEXT NOT NULL, `Reference` TEXT NOT NULL, `SymbolId` INTEGER NOT NULL, `Local` INTEGER NOT NULL)");

i.e.

`Reference` TEXT NOT NULL,