How to implement complex queries using a Content Provider?

Question

I am asking this because I am not quite sure of how to work with Android Content Providers. I have a subset of my database with 8 tables and I need to create complex queries to get some of the data. My content provider works fine with simple queries. For example, I have a table Person on my PersonModel.java class and I get the data using:

String [] projection = {PersonModel.C_FIRST_NAME, PersonModel.C_LAST_NAME};
Cursor cursor = context.getContentResolver().query(
            MyProvider.CONTENT_URI_PERSONS, projection, null,
            null, null);

and it works perfectly.

On MyProvider I have a bunch of CONTENT_URI constants, on for each of my tables.

public class MyProvider extends ContentProvider {
    MyDbHelper dbHelper;
    SQLiteDatabase db;


    private static final String AUTHORITY = "com.myapp.models";

    //Paths for each tables
    private static final String PATH_PROFILE_PICTURES = "profile_pictures";
    private static final String PATH_PERSONS = "persons";
    private static final String PATH_USERS = "users";
    ....

    //Content URIs for each table
    public static final Uri CONTENT_URI_PROFILE_PICTURES = Uri
        .parse("content://" + AUTHORITY + "/" + PATH_PROFILE_PICTURES);
    public static final Uri CONTENT_URI_PERSONS = Uri.parse("content://"
        + AUTHORITY + "/" + PATH_PERSONS);
    public static final Uri CONTENT_URI_USERS = Uri.parse("content://"
        + AUTHORITY + "/" + PATH_USERS);
    ...


    private static final int PROFILE_PICTURES = 1;
    private static final int PROFILE_PICTURE_ID = 2;
    private static final int PERSONS = 3;
    private static final int PERSON_ID = 4;
    private static final int USERS = 5;
    private static final int USER_ID = 6;

    private static final UriMatcher sURIMatcher = new UriMatcher(
        UriMatcher.NO_MATCH);
    static {
    sURIMatcher.addURI(AUTHORITY, PATH_PROFILE_PICTURES, PROFILE_PICTURES);
    sURIMatcher.addURI(AUTHORITY, PATH_PROFILE_PICTURES + "/#",
            PROFILE_PICTURE_ID);
    sURIMatcher.addURI(AUTHORITY, PATH_PERSONS, PERSONS);
    sURIMatcher.addURI(AUTHORITY, PATH_PERSONS + "/#", PERSON_ID);
    sURIMatcher.addURI(AUTHORITY, PATH_USERS, USERS);
    sURIMatcher.addURI(AUTHORITY, PATH_USERS + "/#", USER_ID);
    ...
    }



  public Cursor query(Uri uri, String[] projection, String selection,
        String[] selectionArgs, String sortOrder) {

    // Uisng SQLiteQueryBuilder instead of query() method
    SQLiteQueryBuilder queryBuilder = new SQLiteQueryBuilder();

    // check if the caller has requested a column which does not exists
    //checkColumns(projection);

    int uriType = sURIMatcher.match(uri);

    switch (uriType) {
    case PROFILE_PICTURES:
        queryBuilder.setTables(ProfilePictureModel.TABLE_PROFILE_PICTURE);
        break;
    case PROFILE_PICTURE_ID:
        // Adding the ID to the original query
        queryBuilder.appendWhere(ProfilePictureModel.C_ID + "="
                + uri.getLastPathSegment());
    case PERSONS:
        queryBuilder.setTables(PersonModel.TABLE_PERSON);
        break;
    case PERSON_ID:
        // Adding the ID to the original query
        queryBuilder.appendWhere(PersonModel.C_ID + "="
                + uri.getLastPathSegment());
    case USERS:
        queryBuilder.setTables(UserModel.TABLE_USER);
        break;
    case USER_ID:
        // Adding the ID to the original query
        queryBuilder.appendWhere(UserModel.C_ID + "="
                + uri.getLastPathSegment());

    default:
        throw new IllegalArgumentException("Unknown URI: " + uri);

    }

    db = dbHelper.getWritableDatabase();
    Cursor cursor = queryBuilder.query(db, projection, selection,
            selectionArgs, null, null, sortOrder);
    // make sure that potential listeners are getting notified
    cursor.setNotificationUri(getContext().getContentResolver(), uri);

 }

That is a small part of my content provider. So my questions are:

1) How do I implement a rawQuery() in my content provider? or how do I use properly my queryBuilder?, let's say I want to execute this query using several tables, renaming them and also passing the p1.id as a parameter?

SELECT p1.first_name, p1_last_name 
FROM Person p1, Person P2, Relationship r 
WHERE p1.id = ? AND 
      p1.id = r.relative_id AND
      p2.id = r.related_id;

I tried so by doing this: On my query() method (shown above) I have a new case, called GET_RELATIVES:

case GET_RELATIVES:
        db = dbHelper.getWritableDatabase();
        queryBuilder.setTables(PersonModel.TABLE_PERSON + " p1, "
                + PersonModel.TABLE_PERSON + " p2, "
                + RelationshipModel.TABLE_RELATIONSHIP + " r");
        queryBuilder.appendWhere("p2."+PersonModel.C_ID + "=" + uri.getLastPathSegment());
        queryBuilder.appendWhere("p2."+PersonModel.C_ID + "=" + "r.related_id");
        queryBuilder.appendWhere("p1."+PersonModel.C_ID + "=" + "r.relative_id");

so I defined a new PATH, CONTENT URI and add it to the UriMatcher, like this:

private static final String PATH_GET_RELATIVES = "get_relatives";
public static final Uri CONTENT_URI_GET_RELATIVES = Uri
        .parse("content://" + AUTHORITY + "/"
                + PATH_GET_RELATIVES);
private static final int GET_RELATIVES = 22;

private static final UriMatcher sURIMatcher = new UriMatcher(
    UriMatcher.NO_MATCH);
static {
...
sURIMatcher.addURI(AUTHORITY, PATH_GET_RELATIVES, GET_RELATIVES);
}

but this does not seem to work so I think I'm probably defining something wrong on my content provider or inside the query method.

2) I am not quite sure what is the point on having for each table a constant called TABLE_ID and adding it to the switch-case. What is that used for? How do I call it?

Hope anyone can help me with this, thanks in advance!

Answer 1

I actually found the answer to my question in the most obvious place: the android documentation.

First Question: Implement a rawQuery. Did it like this:

Inside of my switch-case in the content provider I added a new URI, which for me is a JOIN between to tables, so I created a new ContentUri constant for it, a new ID, and registered it on the UriMatcher and then wrote the rawQuery. So MyProvider now looks a litte bit like this:

public class MyProvider extends ContentProvider {
...
// JOIN paths
    private static final String PATH_RELATIONSHIP_JOIN_PERSON_GET_RELATIVES = 
            "relationship_join_person_get_relatives";
...
public static final Uri CONTENT_URI_RELATIONSHIP_JOIN_PERSON_GET_RELATIVES = Uri
            .parse("content://" + AUTHORITY + "/"
                    + PATH_RELATIONSHIP_JOIN_PERSON_GET_RELATIVES);
...
    private static final int RELATIONSHIP_JOIN_PERSON_GET_RELATIVES = 21;
private static final UriMatcher sURIMatcher = new UriMatcher(
            UriMatcher.NO_MATCH);
    static {
...
//JOINS
        sURIMatcher.addURI(AUTHORITY, PATH_RELATIONSHIP_JOIN_PERSON_GET_RELATIVES + "/#",
                RELATIONSHIP_JOIN_PERSON_GET_RELATIVES);
...

public Cursor query(Uri uri, String[] projection, String selection,
            String[] selectionArgs, String sortOrder) {

        // Uisng SQLiteQueryBuilder instead of query() method
        SQLiteQueryBuilder queryBuilder = new SQLiteQueryBuilder();

        // check if the caller has requested a column which does not exists
        //checkColumns(projection);

        int uriType = sURIMatcher.match(uri);

        switch (uriType) {
        ...
        case RELATIONSHIP_JOIN_PERSON_GET_RELATIVES:
            db = dbHelper.getWritableDatabase();
            String[] args = {String.valueOf(uri.getLastPathSegment())};
            Cursor cursor = db.rawQuery(
                    "SELECT p1.first_name, p1.last_name " +
                    "FROM Person p1, Person p2, Relationship r " +
                    "WHERE p1.id = r.relative_id AND " +
                    "p2.id = r.related_id AND " + 
                    "p2.id = ?", args);
            cursor.setNotificationUri(getContext().getContentResolver(), uri);
            return cursor;
        ...
}

And to call the query() method and pass the id ad a parameter I did this in my controller:

String[] projection = { PersonModel.C_FIRST_NAME,
                PersonModel.C_LAST_NAME };
        Cursor cursor = context.getContentResolver().query(
                ContentUris.withAppendedId(
                        AkdemiaProvider.CONTENT_URI_RELATIONSHIP_JOIN_PERSON_GET_RELATED, id), 
                        projection, null, null, null);

Second question: Having the TABLE_ID constant is useful to have a query for each table passing an id as a parameter, I didn't know how to call the query method passing such id and this is how the Android Developer Documentation explains how to do so using ContentUris.withAppendedId

// Request a specific record.


Cursor managedCursor = managedQuery(
                ContentUris.withAppendedId(Contacts.People.CONTENT_URI, 2),
                projection,    // Which columns to return.
                null,          // WHERE clause.
                null,          // WHERE clause value substitution
                People.NAME + " ASC");   // Sort order.

I you guys want to see the whole documentation go to this link.

Hope this helps to anyone else having the same problem to understand ContentProvider, ContentUris and all that :)

Answer 2

Below code worked for me. Inside your Application's Content Provider:

public static final String PATH_JOIN_TWO_TABLES = "my_path";

    public static final Uri URI_JOIN_TWO_TABLES =
            Uri.parse("content://" + AUTHORITY + "/" + PATH_JOIN_TWO_TABLES);

    private static final int ID_JOIN_TWO_TABLES = 1001;

    private static final UriMatcher sURIMatcher = new UriMatcher(
            UriMatcher.NO_MATCH);

    static {
        sURIMatcher.addURI(AUTHORITY,
                PATH_JOIN_TWO_TABLES + "/#", ID_JOIN_TWO_TABLES );
    }

    @Nullable
    @Override
    public Cursor query(@NonNull Uri uri, String[] projection, String selection,String[] selectionArgs,
                        String sortOrder, CancellationSignal cancellationSignal) {

        int uriType = sURIMatcher.match(uri);
            switch (uriType) {

                case ID_JOIN_TWO_TABLES:
                    return getWritableDatabase()
                            .rawQuery("select * from " +
                                    "table_one" + " LEFT OUTER JOIN "
                                    + "table_two" + " ON ("
                                    + "table_one.ID"
                                    + " = " + "table_two.id" + ")", null);
            }
        return super.query(uri, projection, selection, selectionArgs, sortOrder, cancellationSignal);
    }

And while making the Query inside your Activity or Fragment:

 Cursor cursor = getActivity().getContentResolver()
                .query(ContentUris.withAppendedId(MYContentProvider.URI_JOIN_TWO_TABLES, MyContentProvider.ID_JOIN_TWO_TABLES), null, null, null, null);

Hope it works for you.