我问这个是因为我不太确定如何使用 Android 内容提供程序。我有一个包含 8 个表的数据库子集，我需要创建复杂的查询来获取一些数据。我的内容提供者可以很好地处理简单的查询。例如，我的 PersonModel.java 类中有一个 Person 表，我使用以下方法获取数据:
String [] projection = {PersonModel.C_FIRST_NAME, PersonModel.C_LAST_NAME};
Cursor cursor = context.getContentResolver().query(
MyProvider.CONTENT_URI_PERSONS, projection, null,
null, null);
而且效果很好。
在 MyProvider 上，我有一堆 CONTENT_URI 常量，用于我的每个表。
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);
}
那是我的内容提供者的一小部分。所以我的问题是:
1) 如何在我的内容提供者中实现 rawQuery()？或者我如何正确使用我的 queryBuilder？假设我想使用多个表执行此查询，重命名它们并将 p1.id 作为参数传递？
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;
我尝试这样做:在我的 query() 方法(如上所示)中，我有一个新案例，称为 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");
所以我定义了一个新的 PATH，CONTENT URI 并将其添加到 UriMatcher，如下所示:
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);
}
但这似乎不起作用，所以我想我可能在我的内容提供者或查询方法中定义了错误。
2) 我不太确定为每个表设置一个名为 TABLE_ID 的常量并将其添加到 switch-case 有什么意义。那是做什么用的？我怎么调用它？
希望有人能帮我解决这个问题，在此先感谢!
最佳答案
我实际上在最明显的地方找到了我的问题的答案:android 文档。
第一个问题:实现一个rawQuery。是不是这样:
在内容提供者的 switch-case 中，我添加了一个新的 URI，对我来说它是表之间的 JOIN，所以我为它创建了一个新的 ContentUri 常量，一个新的 ID，并将它注册到 UriMatcher 和然后写了rawQuery。所以 MyProvider 现在看起来有点像这样:
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;
...
}
为了调用 query() 方法并传递 id 广告参数，我在我的 Controller 中这样做了:
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);
第二个问题:拥有 TABLE_ID 常量对于查询每个传递 id 作为参数的表很有用，我不知道如何调用传递此类 id 的查询方法，这就是 Android 开发者文档解释的方式使用 ContentUris.withAppendedId
这样做// 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.
我想看整个文档去 this link.
希望这有助于其他有同样问题的人理解 ContentProvider、ContentUris 和所有这些:)
关于java - 如何使用 Content Provider 实现复杂的查询？，我们在Stack Overflow上找到一个类似的问题： https://stackoverflow.com/questions/25210359/