Different accounts with different content, managing user preferences

Question

On the bottom I added a picture of my current app structure and the current code for included data classes / entities.

At the moment in my app the user inserts the url and the code in the Login Fragment, clicking on save button the request to get the token starts. When successful the token is passed to the other requests to fetch the categories data. The different categories I get from the response are then showed in a recyclerview. By clicking on a category the user comes to the movies / seriers by genre Fragment, there I have another recyclerview with the list of movies or series.

When the token-request is successful the url & code are send also to a data class (entity) called AccountData, additional there is a unique String, put together of the url and the code, which works as Primary Key. The AccountData is shown in a recyclerview on the Account Managment Fragment, which is the start screen of the app. Now I want to give the user the option to select for each account, the categories he wants to have shown. Having the possibility to modify he's preferences everytime he wants. For example:

AccountA has 10 movie categories, the user wants have shown only 5 of them. AccountB has 15 movie categories, the user wants have shown only 6 of them.

My idea is to create a new Fragment, MovieCategorySelectFragment or so, where the user can click the categories he wants, passing the selected categories to the Movies Categories Fragment, like a Favorite-List. For implementing this I think about Room. So I made the MovieCategory data class an Entity, using "Id" as Primary Key, and then, considering that it is a one to many relationship (I hope I am right with this), I added the Primary Key from the AccountData Entity to the MovieCategory Entity. I made the String nullable -> val accountData: String?, that I don't get the NullpointerException error.

But now I'm stuck, would it be better to create a new data class / entity, calling it fe SelectedMovieCategory, and passing to it the selected item/category (from the MovieCategorySelectFragment, which is not part of a database) to it and using the room database then do display the selectet categories in the Movies Categories Fragment. Or should I make the request for the categories and save them immediately in the room-database and handling then the process of selecting?

And finally, on both methods, how can I pass the primary key from AccountData to MovieCategory? Otherwise there is no relationship between them? Do I have to create a function in the Dao to handle this?

At the end in the Account Management Fragment the user should be able to click on the account he wants to load, having loaded for each account only the categories he selected before. With the ability to change his preferences going into the MovieCategorySelectFragment and add or remove some categories from his "favorite-list".

Hopefully someone can help me to find the best and easiest way to handle this.

These are the data classes:

data class MovieCategoryResponse(
    val js: List<MovieCategory>
)

@Entity
@Parcelize
data class MovieCategory(
    @PrimaryKey(autoGenerate = false)
    val id: String,
    val number: Int,
    val title: String,
    
    val accountData: String? 
) : Parcelable


@Entity
data class AccountData(
    val url: String,
    val code: String,
    @PrimaryKey(autoGenerate = false)
    val totalAccountData: String
)

Answer 1

@MikeT

Sorry for the late response...(I am using an answer because It's to long for a comment and editing my question would be messy) I was nearly able to get everything I wanted, using this way (example series categories):

I saved the fetched categories (using retrofit) in a new POJO, named SeriesCategory:

@Entity(
    foreignKeys = [
        ForeignKey(
            entity = AccountData::class,
            parentColumns = ["totalAccountData"],
            childColumns = ["accountData"],
            onDelete = ForeignKey.CASCADE,
            onUpdate = ForeignKey.CASCADE,
        )
    ]
)
@Parcelize
data class SeriesCategory(
    @PrimaryKey(autoGenerate = false)
    val id: String,
    val title: String,
    @ColumnInfo(index = true)
    var accountData: String,
    var favorite: Boolean,
    val idByAccountData: String
) : Parcelable

using...

@Insert(onConflict = OnConflictStrategy.REPLACE)
    suspend fun insertSeriesCategory(seriescategory: List<SeriesCategory>)

                val mappedSeriesCategoryList =   seriescatresponse.js.map { SeriesCategorybyAccount(it.id, it.title, totalAccountData, isFavorite, "${it.id}$totalAccountData") }
               lifecycleScope.launch {
                   mappedSeriesCategoryList.forEach { dao.insertSeriesCategory(mappedSeriesCategoryList) }
               }

In the Dao & SeriesCategoryFragment I use following code to get all categories from this specific account:

@Query("SELECT * FROM seriesCategory WHERE accountData=:totalAccountData")
    fun getSeriesCategoriesPerAccount(totalAccountData: String): LiveData<List<SeriesCategory>>

        viewModel.getSeriesCategoryByAccount(totalAccountData, this@SeriesCategoryFragment.requireActivity()).observe(viewLifecycleOwner) {
            seriesCategoryAdapter.submitList(it)
        }

As I am using a checkbox in the SeriesCategoryFragment-recyclerview I managed the checkbox in the adapter and used following code in the Dao & SelectedSeriesCategoryFragment:

@Query("SELECT * FROM seriesCategory WHERE accountData=:totalAccountData AND favorite = 1")
    fun getSelectedSeriesCategoriesPerAccount(totalAccountData: String): LiveData<List<SeriesCategory>>

        viewModel.getSelectedSeriesCategoryByAccount(totalAccountData, this@SeriesSelectedFragment.requireActivity()).observe(viewLifecycleOwner) {
            selectedSeriesCategoryAdapter.submitList(it)
        }

Thats working fine, when in the SeriesCategoryFragment a checkbox is checked(favorite = true), the category is "send" to the SelectedSeriesCategoryFragment, if it's unchecked (favorite = false) it's removed.

But at the moment I am not able to save this in my room database. So when I have for example following categories: CategoryA, CategoryB, CategoryC, CategoryD

The user checks CategoryA and CategoryC using the checkbox, this two categories are then displayed in the SelectedSeriesCategoryFragment-recyclerview. When he unchecks the checkbox the category is removed. That's all ok.

In the same clicklistener I am using also @Update with (it = SeriesCategory):

viewModel.updateSeriesCategory(it, this@SeriesCategoryFragment.requireActivity())

Restarting the app the database shows me favorite = 1 (for true) on the categories that where checked before the restart. But the checkboxes are unchecked - an the @Query I use in the SelectedSeriesCategoryFragment (see above) doesn't seem to work anymore -> means, it is empty. On a second restart the database shows all categories as favorite = 0 (for false) - probably because the checkbox was empty before the second restart. So as I understand, the idea I have should work - but only if the checked Checkboxes stay checked also after the restart. Can I handle this somehow with room?

Or is this a completely recyclerview-specific problem?

Eventually, my Viewholder in the Adapter looks like this:

inner class ViewHolder(val binding: RvItemSeriescategoryBinding) : RecyclerView.ViewHolder(binding.root) {
        fun bind(category: SeriesCategory) {
            binding.apply {
                rvItemSeriescategory.text = category.title
                checkboxSeriescategory.isChecked = category.favorite
                checkboxSeriescategory.setOnClickListener {
                        if (checkboxSeriescategory.isChecked) {
                        category.favorite = true
                            onClickListener.onClick(category)
                        }
                    if (!checkboxSeriescategory.isChecked)
                        category.favorite = false
                        onClickListener.onClick(category)
                }
                if (category.favorite == true){
                    checkboxSeriescategory.isChecked = true
                }
            }
        }
    }

Answer 2

It appears that you want accounts, moviecategories, seriescategories and a means of associating/relating accounts with a number of moviecategories and perhaps seriescategories and to be able to restrict the number of listed moviecategories according to an accounts preference which can change.

The solution would be a many-many relationship between account and moviecategories (and perhaps seriescategories).

However, before moving on you say:-

So I made the MovieCategory data class an Entity, using "Id" as Primary Key, and then, considering that it is a one to many relationship (I hope I am right with this), I added the Primary Key from the AccountData Entity to the MovieCategory Entity.

Regarding your hope. I believe that you are wrong. There are 3 types of relationships:-

1. 1-1 where each row in a table would have a means of uniquely id entifying the row, typically via a single column, in other table (if a separate table is apt (a single table may well suffice)). 1-1 relationships are not typically catered for by using tables.

2. 1-many where a row in the parent table (account) could have many children (moviecategory) BUT the moviecategory could only have the 1 parent. In such a case a column in the moviecategory would contain a value that uniquely id entifies the parent.

3. many-many an expanded 1-M that allows each side to relate to any number of the other side. So an account can relate to many moviecategories to which other accounts can relate to. The typical solution is to have an intermediate table that has 2 core columns. One that stores the value that uniquely id entifies one side of the relationship and the other that stores the value that uniquely id entifies the other side. Typically the 2 columns would for the primary key.

   • such an intermediate table has numerous terms to describe such a table, such as associative table, mapping table, reference table....

• Note how id has been highlighted. Simply creating a column called id in a table (Entity) does not make a relationship, it only supports the potential of a relationship being made.

You problem would appear to tick the boxes for a many-many relationship and thus the extra table (2 if account-secriescategories).

This table would have a column for the the value that uniquely identifies the accountData row ( totalAccountData ).

• as the totalAccountData is the primary key (ie it is annotated with @PrimaryKey) and that a PrimaryKey is implicitly unique

The table would have a second column for the movieCategory's id column.

So you could start with

@Entity
data class AccountMovieMap(
    val accountDataMap: String,
    val movieCategoryMap: String
) 

However, there is no PrimaryKey which room requires BUT the @PrimaryKey annotation only applies to a single column. If either were used then due to the implicit uniqueness the relationship would be restricted to 1-many. A composite (multiple columns/values) Primary Key is required that makes the uniqueness according to the combined values. To specify a composite PrimaryKey in Room the primaryKeys parameter of the @Entity annotation is used.

So AccountMovieMap becomes:-

@Entity(
    primaryKeys = ["accountDataMap","movieCategoryMap"]
)
data class AccountMovieMap(
    val accountDataMap: String,
    val movieCategoryMap: String
)

As it stands there is a potential issue with the above as it is possible to insert data into either or both columns that is not a value in the respective table. That is the integrity of the relationship, in such a situation, does not exist.

SQLite and therefore Room (as with many relational database) caters for enforcing Referential Integrity . SQLite does this via ForeignKey clauses. Room uses the foreignKeys parameter of the @Entity annotation to provide a list of ForeignKey s.

• In addition to enforcing referential integrity SQlite has 2 clauses ON DELETE and ON UPDATE that help to maintain referential integrity (depending upon the specified action, the most useful being CASCADE which allows changes that would break referential integrity by applying changes to the parent to the children).

• Room will also warn if an index does not exist where it believe one should eg warning: movieCategoryMap column references a foreign key but it is not part of an index. This may trigger full table scans whenever parent table is modified so you are highly advised to create an index that covers this column. warning: movieCategoryMap column references a foreign key but it is not part of an index. This may trigger full table scans whenever parent table is modified so you are highly advised to create an index that covers this column. As such, the @ColumnInfo annotation can be used to add an index on the movieCategoryMap column.

So AccountMovieMap could be the fuller:-

@Entity(
    primaryKeys = ["accountDataMap","movieCategoryMap"]
    , foreignKeys = [
        ForeignKey(
            entity = AccountData::class,
            parentColumns = ["totalAccountData"],
            childColumns = ["accountDataMap"],
            /* Optional but helps to maintain Referential Integrity */
            onDelete = ForeignKey.CASCADE,
            onUpdate = ForeignKey.CASCADE
        ),
        ForeignKey(
            entity = MovieCategory::class,
            parentColumns = ["id"],
            childColumns = ["movieCategoryMap"],
            onDelete = ForeignKey.CASCADE,
            onUpdate = ForeignKey.CASCADE
        )
    ]
)
data class AccountMovieMap(
    val accountDataMap: String,
    @ColumnInfo(index = true)
    val movieCategoryMap: String
)

To add (insert) rows you could then have/use (in an @Dao annotated class):-

@Insert(onConflict = OnConflictStrategy.IGNORE)
fun insert(accountMovieMap: AccountMovieMap)

• noting that to avoid referential integrity conflicts that the accountData referenced/mapped and MovieCategory referenced/mapped need to exist.

As you would want to extract an AccountData's MovieCategories you need a POJO that has the AccountData with a List of MovieCategory.

This could be:-

data class AccountWithMovieCategoryList(
    @Embedded
    val accountData: AccountData,
    @Relation(
        entity = MovieCategory::class,
        parentColumn = "totalAccountData", /* The column referenced in the @Embedded */ 
        entityColumn = "id", /* The column referenced in the @Relation (MovieCategory) */
        /* The mapping table */
        associateBy = (
                Junction(
                    value = AccountMovieMap::class, /* The @Entity annotated class for the mapping table */
                    parentColumn = "accountDataMap", /* the column in the mapping table that references the @Embedded */
                    entityColumn = "movieCategoryMap" /* the column in the mapping table that references the @Relation */
                )
                )
    )
    val movieCategoryList: List<MovieCategory>
)

The following could be the function in an @Dao annotated interface that retrieves an AccountWithMovieCategoryList for a given account:-

@Transaction
@Query("SELECT * FROM accountData WHERE totalAccountData=:totalAccountData")
fun getAnAccountWithMovieCategoryList(totalAccountData: String): List<AccountWithMovieCategoryList>

However Room will retrieve ALL the MovieCategories but you want to be able to specify a LIMIT ed number of MovieCategories for an Account, so a means is required to override Room's methodology of getting ALL mapped/associate objects.

To facilitate this then a function with a body can be used to a) get the respective AccountData and b) to then get the MovieCategory list according to the account, via the mapping table with a LIMIT specified. Thus 2 @Query functions to doe the 2 gets invoked by the overarching function.

So to get the AccountData:-

@Query("SELECT * FROM accountData WHERE totalAccountData=:totalAccountData")
fun getSingleAccount(totalAccountData: String): AccountData

And then to get the limited MovieCategories for an AccountData via (JOIN) the mapping table:-

@Query("SELECT movieCategory.* FROM accountMovieMap JOIN movieCategory ON accountMovieMap.MovieCategoryMap = movieCategory.id WHERE accountMovieMap.accountDataMap=:totalAccountData LIMIT :limit")
fun getLimitedMovieCategoriesForAnAccount(totalAccountData: String,limit: Int): List<MovieCategory>

And to put it all together aka the overarching function:-

@Transaction
@Query("")
fun getAccountWithLimitedMovieCategoryList(totalAccountData: String,categoryLimit: Int): AccountWithMovieCategoryList {
    return AccountWithMovieCategoryList(
        getSingleAccount(totalAccountData),
        getLimitedMovieCategoriesForAnAccount(totalAccountData,categoryLimit)
    )
}

• Note the above code has only been compiled (so Room processing sees no issues), as such it is in-principle code

• You say Best , this is opiniated and is not the best as a better way would be to utilise SQLite's more efficient handling of INTEGER primary keys.

• categoryLimit ie an Int passed could be dynamically via the UI selected or stored and thus preserved. It could be stored in the AccountData by adding a suitable column or it could be stored elsewhere. AccountData would seem the simplest and most apt unless the expectation is that many such account based preferences should exist.

If, for example, an extra column was added to AccountData eg :-

@Entity
data class AccountData(
    val url: String,
    val code: String,
    @PrimaryKey(autoGenerate = false)
    val totalAccountData: String,
    val movieCategoryLimit: Int /*<<<<< to store LIMIT preference */
)

A means of changing the limit would very likely be required such as the following in a/the @Dao annotated interface:-

@Query("UPDATE accountData SET movieCategoryLimit=:newLimit WHERE totalAccountData=:totalAccountData")
fun changeMovieCategoryLimit(totalAccountData: String, newLimit: Int)

A subtle change to the getAccountWithLimitedMovieCategoryList function and the LIMIT is as per the preference:-

@Transaction
@Query("")
fun getAccountWithLimitedMovieCategoryList(totalAccountData: String,categoryLimit: Int): AccountWithMovieCategoryList {
    val accountData = getSingleAccount(totalAccountData)
    return AccountWithMovieCategoryList(
        accountData,
        getLimitedMovieCategoriesForAnAccount(totalAccountData,accountData.movieCategoryLimit)
    )
}

• ie rather than using the retrieved AccountData directly, it is retrieved into a val, the val is then used to provide the AccountData and then again to provide the value for the LIMIT.

Additional

As per the comment

.... Isn't it then a one-to-many relation? ....

Then for a 1-M, as previously explained MovieCategory should have a column for storing a unique column. Which I guess is what you index the val accountData: String? to be for.

• The ? should never be null (an orphan that is basically useless). Ideally, as the column use would imply selection via this column, the column should be indexed. As the intended use is as a foreign key, then although not required defining it as a foreign and enforcing referential integrity makes sense (and also goes towards describing/comment the column as a foreign key). Then the MovieCategory class could be

:-

/* MovieCategory modified for 1-m (1 account many categories)*/
@Entity(
    foreignKeys = [
        ForeignKey(
            entity = AccountData::class,
            parentColumns = ["totalAccountData"],
            childColumns = ["accountData"],
            onDelete = ForeignKey.CASCADE,
            onUpdate = ForeignKey.CASCADE,
        )
    ]
)
@Parcelize
data class MovieCategory(
    @PrimaryKey(autoGenerate = false)
    val id: String,
    val number: Int,
    val title: String,
    @ColumnInfo(index = true) /* added as likely to used for selecting rows */
    val accountData: String /* should NEVER by null */
) : Parcelable

Assuming that AccountData includes the movieCategoryLimit (as explained above) then a POJO for retrieving the Account and the LIMITED list of MovieCategories would be required (a little different to the mm equivalent) which could be:-

data class AccountDataWithMovieCategories(
    @Embedded
    val accountData: AccountData,
    @Relation(
        entity = MovieCategory::class,
        parentColumn = "totalAccountData",
        entityColumn = "accountData"
    )
    val movieCategories: List<MovieCategory>
)

• ie no association table involved

The same issue exists that Room will retrieve ALL MovieCategories. So the following is not what you want:-

/* Note will get ALL MovieCategories (even with join and LIMIT) */
@Transaction
@Query("SELECT * FROM accountData WHERE totalAccountData=:totalAccountData")
fun getAccountWithMovieCategories(totalAccountData: String): List<AccountWithMovieCategoryList>

Rather you want to have:-

@Query("SELECT * FROM movieCategory WHERE accountData=:totalAccountData LIMIT :limit")
fun getLimitedMoviesCategoriesPerAccount(totalAccountData: String, limit: Int): List<MovieCategory>

• ie directly accessing the MovieCategory rather than via the JOIN on the association table in the mm version

An then finally the overarching function:-

@Transaction
@Query("")
fun getAccountWithLimitedMovieCategories(totalAccountData: String, limit: Int): AccountDataWithMovieCategories {
    val accountData = getSingleAccount(totalAccountData)
    return AccountDataWithMovieCategories(
        accountData,
        getLimitedMovieCategoriesForAnAccount(
            accountData.totalAccountData,
            accountData.movieCategoryLimit /* the limit as stored in the account */
        )
    )
}