Confused about _ID (ContentProviders)

mangaluve · by **mangaluve** » Thu Mar 26, 2009 11:23 am

Im reading about ContentProviders, but something is confusing me. All the docs says that I have to have a field called _ID. But where do I need this field to have exactly this name? For instance if I store my data in a database. Is the database required to have a field with exactly that name? Or can I call it whatever I want? Where is exactly that name needed?

WarrenFaith · by **WarrenFaith** » Thu Mar 26, 2009 11:38 am

Check the class BaseColumns.
I implement it with every table I use so I dont have to care about the primary id

mangaluve · by **mangaluve** » Thu Mar 26, 2009 12:37 pm

I don't really understand what you mean. Where do you use it and how (BaseColumns). Is the _ID field required at all (with that exact name)? Couldn't I have a database where the id of a row is called MYID instead? is it somethings pecial with the name _ID?

WarrenFaith · by **WarrenFaith** » Thu Mar 26, 2009 1:15 pm

Sorry, i am on my way out, but here is my provider and my data classes. Simply inspired by the notepad tutorial:

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package com.droidnova.android.lineofsight.data;
 
import android.net.Uri;
 
import android.provider.BaseColumns;
 
/**
 
 * Convenience definitions for PlayerProvider.
 
 * 
 
 * @author Martin
 
 */
 
public final class LineOfSightData {
 
    public static final String AUTHORITY = "com.droidnova.android.lineofsight.provider.LineOfSightData";
 
    /**
 
     * The name of the database file.
 
     */
 
    public static final String DATABASE_NAME = "avatar.db";
 
    /**
 
     * User table.
 
     * 
 
     * @author Martin
 
     */
 
    public static final class Player implements BaseColumns {
 
        private Player() {
 
        }
 
        /**
 
         * The content:// style URL for this table
 
         */
 
        public static final Uri CONTENT_URI = Uri.parse("content://" + AUTHORITY + "/avatar");
 
        /**
 
         * The MIME type of {@link #CONTENT_URI} providing a directory of users.
 
         */
 
        public static final String CONTENT_TYPE = "vnd.android.cursor.dir/vnd.lineofsight.avatar";
 
        /**
 
         * The MIME type of a {@link #CONTENT_URI} sub-directory of a single
 
         * user.
 
         */
 
        public static final String CONTENT_ITEM_TYPE = "vnd.android.cursor.item/vnd.lineofsight.avatar";
 
        /**
 
         * The default sort order for this table
 
         */
 
        public static final String DEFAULT_SORT_ORDER = "id DESC";
 
        /**
 
         * The name of the character.
 
         */
 
        public static final String NAME = "name";
 
        /**
 
         * The gender of the character.
 
         */
 
        public static final String GENDER = "gender";
 
        /**
 
         * The Race of the character.
 
         */
 
        public static final String RACE = "race";
 
        /**
 
         * The level the character currently is.
 
         */
 
        public static final String LEVEL = "level";
 
        /**
 
         * The amount of experience the character has.
 
         */
 
        public static final String EXPERIENCE = "experience";
 
        /**
 
         * The amount of health the character has.
 
         */
 
        public static final String CURRENT_HEALTH = "current_health";
 
        /**
 
         * The amount of witcheries the character currently has.
 
         */
 
        public static final String CURRENT_WITCHERIES = "current_witcheries";
 
        /**
 
         * The strength the character has.
 
         */
 
        public static final String STRENGTH = "strength";
 
        /**
 
         * The agility the character has.
 
         */
 
        public static final String AGILITY = "agility";
 
        /**
 
         * The life the character has.
 
         */
 
        public static final String LIFE = "life";
 
        /**
 
         * The witchcraft the character has.
 
         */
 
        public static final String WITCHCRAFT = "witchcraft";
 
        /**
 
         * Free points to assign to other things.
 
         */
 
        public static final String ATTRIBUTE_POINTS = "attribute_points";
 
    }
 
}
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package com.droidnova.android.lineofsight.provider;
 
import java.util.HashMap;
 
import android.content.ContentProvider;
 
import android.content.ContentUris;
 
import android.content.ContentValues;
 
import android.content.Context;
 
import android.content.UriMatcher;
 
import android.database.Cursor;
 
import android.database.sqlite.SQLiteDatabase;
 
import android.database.sqlite.SQLiteOpenHelper;
 
import android.net.Uri;
 
import android.text.TextUtils;
 
import android.util.Log;
 
import com.droidnova.android.lineofsight.data.LineOfSightData;
 
import com.droidnova.android.lineofsight.data.LineOfSightData.Player;
 
/**
 
 * Entity class for a database with one table.
 
 * 
 
 * @author Martin
 
 */
 
public class AvatarProvider extends ContentProvider {
 
    /* Tag for logging */
 
    private static final String TAG = "AvatarProvider";
 
    private static final int CHARACTER = 1;
 
    private static final int CHARACTER_ID = 2;
 
    private static final String DATABASE_TABLE = "avatar";
 
    private static final int DATABASE_VERSION = 1;
 
    private static final UriMatcher uriMatcher;
 
    private static HashMap<String, String> userProjectionMap;
 
    private DatabaseHelper openHelper;
 
    @Override
 
    public boolean onCreate() {
 
        openHelper = new DatabaseHelper(getContext());
 
        return true;
 
    }
 
    @Override
 
    public int delete(Uri uri, String where, String[] whereArgs) {
 
        SQLiteDatabase db = openHelper.getWritableDatabase();
 
        int count;
 
        switch (uriMatcher.match(uri)) {
 
            case CHARACTER:
 
                count = db.delete(DATABASE_TABLE, where, whereArgs);
 
                break;
 
            case CHARACTER_ID:
 
                String playerId = uri.getPathSegments().get(1);
 
                count = db.delete(DATABASE_TABLE, Player._ID + "=" + playerId
 
                        + (!TextUtils.isEmpty(where) ? " AND (" + where + ')' : ""), whereArgs);
 
                break;
 
            default:
 
                throw new IllegalArgumentException("Unknown URI " + uri);
 
        }
 
        getContext().getContentResolver().notifyChange(uri, null);
 
        return count;
 
    }
 
    @Override
 
    public String getType(Uri uri) {
 
        switch (uriMatcher.match(uri)) {
 
            case CHARACTER:
 
                return Player.CONTENT_TYPE;
 
            case CHARACTER_ID:
 
                return Player.CONTENT_ITEM_TYPE;
 
            default:
 
                throw new IllegalArgumentException("Unknown URI " + uri);
 
        }
 
    }
 
    @Override
 
    public Uri insert(Uri uri, ContentValues values) {
 
        // validate the uri
 
        if (uriMatcher.match(uri) != CHARACTER) {
 
            throw new IllegalArgumentException("Unknown URI " + uri);
 
        }
 
        SQLiteDatabase db = openHelper.getWritableDatabase();
 
        long id = db.insert(DATABASE_TABLE, null, values);
 
        if (id == -1) {
 
            return null;
 
        } else {
 
            return ContentUris.withAppendedId(Player.CONTENT_URI, id);
 
        }
 
    }
 
    @Override
 
    public Cursor query(Uri uri, String[] projection, String selection, String[] selectionArgs, String sortOrder) {
 
        SQLiteDatabase db = openHelper.getWritableDatabase();
 
        return db.query(DATABASE_TABLE, projection, selection, selectionArgs, null, null, sortOrder);
 
    }
 
    @Override
 
    public int update(Uri uri, ContentValues values, String where, String[] whereArgs) {
 
        SQLiteDatabase db = openHelper.getWritableDatabase();
 
        int count;
 
        switch (uriMatcher.match(uri)) {
 
            case CHARACTER:
 
                count = db.update(DATABASE_TABLE, values, where, whereArgs);
 
                break;
 
            case CHARACTER_ID:
 
                String playerId = uri.getPathSegments().get(1);
 
                count = db.update(DATABASE_TABLE, values, Player._ID + "=" + playerId
 
                        + (!TextUtils.isEmpty(where) ? " AND (" + where + ')' : ""), whereArgs);
 
                break;
 
            default:
 
                throw new IllegalArgumentException("Unknown URI " + uri);
 
        }
 
        getContext().getContentResolver().notifyChange(uri, null);
 
        return count;
 
    }
 
    static {
 
        uriMatcher = new UriMatcher(UriMatcher.NO_MATCH);
 
        uriMatcher.addURI(LineOfSightData.AUTHORITY, "avatar", CHARACTER);
 
        uriMatcher.addURI(LineOfSightData.AUTHORITY, "avatar/#", CHARACTER_ID);
 
        userProjectionMap = new HashMap<String, String>();
 
        userProjectionMap.put(Player._ID, Player._ID);
 
        userProjectionMap.put(Player.NAME, Player.NAME);
 
        userProjectionMap.put(Player.GENDER, Player.GENDER);
 
        userProjectionMap.put(Player.RACE, Player.RACE);
 
        userProjectionMap.put(Player.LEVEL, Player.LEVEL);
 
        userProjectionMap.put(Player.EXPERIENCE, Player.EXPERIENCE);
 
        userProjectionMap.put(Player.CURRENT_HEALTH, Player.CURRENT_HEALTH);
 
        userProjectionMap.put(Player.CURRENT_WITCHERIES, Player.CURRENT_WITCHERIES);
 
        userProjectionMap.put(Player.STRENGTH, Player.STRENGTH);
 
        userProjectionMap.put(Player.AGILITY, Player.AGILITY);
 
        userProjectionMap.put(Player.LIFE, Player.LIFE);
 
        userProjectionMap.put(Player.WITCHCRAFT, Player.WITCHCRAFT);
 
        userProjectionMap.put(Player.ATTRIBUTE_POINTS, Player.ATTRIBUTE_POINTS);
 
    }
 
    /**
 
     * This class helps open, create, and upgrade the database file.
 
     * 
 
     * @author Martin
 
     */
 
    private static class DatabaseHelper extends SQLiteOpenHelper {
 
        DatabaseHelper(Context context) {
 
            super(context, LineOfSightData.DATABASE_NAME, null, DATABASE_VERSION);
 
        }
 
        @Override
 
        public void onCreate(SQLiteDatabase db) {
 
            db.execSQL("CREATE TABLE " + DATABASE_TABLE + " ("
 
                    + Player._ID + " INTEGER PRIMARY KEY,"
 
                    + Player.NAME + " TEXT,"
 
                    + Player.GENDER + " TEXT,"
 
                    + Player.RACE + " TEXT,"
 
                    + Player.LEVEL + " INTEGER,"
 
                    + Player.EXPERIENCE + " INTEGER,"
 
                    + Player.CURRENT_HEALTH + " INTEGER,"
 
                    + Player.CURRENT_WITCHERIES + " INTEGER,"
 
                    + Player.STRENGTH + " INTEGER,"
 
                    + Player.AGILITY + " INTEGER,"
 
                    + Player.LIFE + " INTEGER,"
 
                    + Player.WITCHCRAFT + " INTEGER,"
 
                    + Player.ATTRIBUTE_POINTS + " INTEGER);");
 
        }
 
        @Override
 
        public void onUpgrade(SQLiteDatabase db, int oldVersion, int newVersion) {
 
            Log.w(TAG, "Upgrading database from version " + oldVersion + " to " + newVersion + ", which will destroy all old data");
 
            db.execSQL("DROP TABLE IF EXISTS " + DATABASE_TABLE);
 
            onCreate(db);
 
        }
 
    }
 
}
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mangaluve · by **mangaluve** » Thu Mar 26, 2009 1:42 pm

Thanks! But I was wondering more about the _ID, do I have to have a field in the (for instance) sql-database with the Exact name _ID? And do I have to have a field (which you inherit from BaseColumns) that has the name _ID? Or could I just name it whatever I want?

Alexandro · by **Alexandro** » Fri Mar 27, 2009 8:42 am

mangaluve wrote:Thanks! But I was wondering more about the _ID, do I have to have a field in the (for instance) sql-database with the Exact name _ID? And do I have to have a field (which you inherit from BaseColumns) that has the name _ID? Or could I just name it whatever I want?

It's not required to have the "_id" field, but without it you won't be able to use CursorAdapter and probably some other things. Having an _id field is only a + to your db.
And no, you can't use some other field. For some things to work you need exactly "_id" field, it doesn't necessary have to be a primary key though.

mangaluve · by **mangaluve** » Fri Mar 27, 2009 10:18 am

Okey thanks.. which are those things? I mean it makes sense to have some kind of ID (of course) and it's nice to always name it _ID by convention, but I want to know where exactly that name is required. I guess my DataBase doesn't need to have a field named exactly ID (since everything goes through the content provider). But where is _ID needed? I understand that some objects assumes that the _ID-fields exists since it's convention, but do I really need it if I write everything by myself and don't use any "helpers" except for ContentProvider and Cursor?

For instance, suppose I implement a simple Phonebook with a textfile (here I cannot really talk about a field named _ID, lets say that every row has a number in front which is the ID). Now I extend ContentProvider and implements the query/insert and so on, and an own Cursor. Where do I need _ID here, do my ContentProvider Have to have that field in order for the android system to accept is as a "shared" ContentProvider or what?

Alexandro · by **Alexandro** » Fri Mar 27, 2009 11:01 am

mangaluve wrote:Okey thanks.. which are those things? I mean it makes sense to have some kind of ID (of course) and it's nice to always name it _ID by convention, but I want to know where exactly that name is required. I guess my DataBase doesn't need to have a field named exactly ID (since everything goes through the content provider). But where is _ID needed? I understand that some objects assumes that the _ID-fields exists since it's convention, but do I really need it if I write everything by myself and don't use any "helpers" except for ContentProvider and Cursor?

For instance, suppose I implement a simple Phonebook with a textfile (here I cannot really talk about a field named _ID, lets say that every row has a number in front which is the ID). Now I extend ContentProvider and implements the query/insert and so on, and an own Cursor. Where do I need _ID here, do my ContentProvider Have to have that field in order for the android system to accept is as a "shared" ContentProvider or what?

SimpleCursorAdapter and CursorAdapter require your cursor (the one you supply to them) to provide data from database with "_id" field.
I had an issue where I was getting a really big database from internet (in binary .sql) and there was no "_db" field. Tried to work it around with SimpleAdapter, but got a lot of lags because of the size, so i had to create "_id" field in it. Not sure where else this field might be needed.

mangaluve · by **mangaluve** » Fri Mar 27, 2009 11:54 am

Thanks!

But for now suppose I don't have a database, I just have a textfile with the info. And I don't want to use a CursorAdaptor or show something, say I just want to print the first retrieved element or something. Is an _ID needed then? I know a lot of the standard classes rely on that and _ID field is present, but is it really Required if I dont use these classes? Or is it possible to do a simple ContentProvider and some kind of Cursor where the "id-field" has another name?

WarrenFaith · by **WarrenFaith** » Fri Mar 27, 2009 12:15 pm

I would say: try it!
I can't say more than: just use _ID... where is the problem to use a convention? It is nearly everywhere needed...

mangaluve · by **mangaluve** » Fri Mar 27, 2009 12:53 pm

Well it's because I just want to know if it's neccesary.. I know I should try but I haven't had the time..it just disturbed me that the document said that I should have a _ID field, then I want to know, what fails if I dont have it. Just the way I learn things. If I want to know why something isn't working, it's not an alternative that Im after

Confused about _ID (ContentProviders)

Confused about _ID (ContentProviders)

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