How do I updated an Android Room column from notNull=true to notNull=false?

The schema in the json file is generated and based upon the Entity, changing it will make no difference. It isn't even required (except if using AutoMigration).

The pre-populated database schema is correct but I cannot get Android Room to update correctly to match:

You have to either change the Entities accordingly or convert the pre-populated database accordingly. Noting again that the Entities define what Room expects.

The language used matters as to the exact answer.

With Kotlin then Notes could be:-

data class Note(
    @PrimaryKey(autoGenerate = true)
    val NoteId: Long,
    val SourceID: Long?,
    val CommentID: Long?,
    val QuestionID: Long?,
    val QuoteID: Long?,
    val TermID: Long, //<<<<< NOT NULL 
    val TopicID: Long?,
    val Deleted: Long?
)

The generated java then shows the table create as :-

_db.execSQL("CREATE TABLE IF NOT EXISTS `Note` (`NoteId` INTEGER PRIMARY KEY AUTOINCREMENT NOT NULL, `SourceID` INTEGER, `CommentID` INTEGER, `QuestionID` INTEGER, `QuoteID` INTEGER, `TermID` INTEGER NOT NULL, `TopicID` INTEGER, `Deleted` INTEGER, FOREIGN KEY(`SourceID`) REFERENCES `Source`(`SourceID`) ON UPDATE NO ACTION ON DELETE NO ACTION , FOREIGN KEY(`CommentID`) REFERENCES `Comment`(`CommentID`) ON UPDATE NO ACTION ON DELETE NO ACTION , FOREIGN KEY(`TopicID`) REFERENCES `Topic`(`TopicID`) ON UPDATE NO ACTION ON DELETE NO ACTION )");

i.e. those with Long? do not have NOT NULL (the TermID column has NOT NULL as Long instead of Long? was used).

With Java the column type cannot be a primitive type for NULLs to be allowed, as these MUST have a value and cannot be null, so Room will derive NOT NULL. Just the object type (e.g. Long not long) will be taken as NULLs allowed. To force NOT NULL then the @NotNull annotation needs to be used.

So Java equivalent (named JavaNote to allow both to be used/compiled) could be :-

@Entity(
        foreignKeys = {
                @ForeignKey(entity = Source.class,parentColumns = {"SourceID"},childColumns = {"SourceID"}),
                @ForeignKey(entity = Comment.class,parentColumns = {"CommentID"},childColumns = {"CommentID"}),
                @ForeignKey(entity = Topic.class,parentColumns = {"TopicID"}, childColumns = {"TopicID"})
        }
)
class JavaNote {
    @PrimaryKey(autoGenerate = true)
    long NoteID=0; // primitives cannot be NULL thus imply NOT NULL
    Long SourceID;
    Long CommentID;
    Long QuestionID;
    Long QuoteID;
    @NotNull
    Long TermID; // or long TermID
    Long TopicID;
    Long Deleted;
}

The generated java then has the table create as :-

_db.execSQL("CREATE TABLE IF NOT EXISTS `JavaNote` (`NoteID` INTEGER PRIMARY KEY AUTOINCREMENT NOT NULL, `SourceID` INTEGER, `CommentID` INTEGER, `QuestionID` INTEGER, `QuoteID` INTEGER, `TermID` INTEGER NOT NULL, `TopicID` INTEGER, `Deleted` INTEGER, FOREIGN KEY(`SourceID`) REFERENCES `Source`(`SourceID`) ON UPDATE NO ACTION ON DELETE NO ACTION , FOREIGN KEY(`CommentID`) REFERENCES `Comment`(`CommentID`) ON UPDATE NO ACTION ON DELETE NO ACTION , FOREIGN KEY(`TopicID`) REFERENCES `Topic`(`TopicID`) ON UPDATE NO ACTION ON DELETE NO ACTION )");

• again the TermID has been purposefully coded to use NOT NULL

The generated java is available after compiling. It is found in the generated java (use Android View) in the member/class name the same as the class annotated with @Database suffixed with _Impl. The statements themselves are in the createAlltables method.

e.g. :-