Develop Reference

How to set a property path in Jena's Sparql API?

I would like to avoid passing SPARQL queries around as Strings. Therefore I use Jena's API for creating my queries. Now I need a PropertyPath in my query, but I can't find any Java class supporting this. Can you give me a hint?
Here's some example code where I would like to insert this (Jena 3.0.1):
private Query buildQuery(final String propertyPath) {
ElementTriplesBlock triplesBlock = new ElementTriplesBlock();
triplesBlock.addTriple(
new Triple(NodeFactory.createURI(this.titleUri.toString()),
//How can I set a property path as predicate here?
NodeFactory.???,
NodeFactory.createVariable("o"))
);
final Query query = buildSelectQuery(triplesBlock);
return query;
}
private Query buildSelectQuery(final ElementTriplesBlock queryBlock) {
final Query query = new Query();
query.setQuerySelectType();
query.setQueryResultStar(true);
query.setDistinct(true);
query.setQueryPattern(queryBlock);
return query;
}

You can use PathFactory to create property paths
Consider the graph below:
#prefix dc: <http://purl.org/dc/elements/1.1/>.
#prefix ex: <http://example.com/>.
ex:Manager ex:homeOffice ex:HomeOffice
ex:HomeOffice dc:title "Home Office Title"
Suppose you want to create a pattern like:
?x ex:homeOffice/dc:title ?title
The code below achieves it:
//create the path
Path exhomeOffice = PathFactory.pathLink(NodeFactory.createURI("http://example.com/homeOffice"));
Path dcTitle = PathFactory.pathLink(NodeFactory.createURI("http://purl.org/dc/elements/1.1/title"));
Path fullPath = PathFactory.pathSeq(exhomeOffice,dcTitle);
TriplePath t = new TriplePath(Var.alloc("x"),fullPath,Var.alloc("title"));

Select with a set of predefined values in JOOQ for PostgreSQL

Does anybody know is it possible to write in JOOQ select with a set of predefined values? I need it for insert if not exists.
For example,
INSERT INTO test
(text)
SELECT '1234567890123456789'
WHERE
NOT EXISTS (
SELECT id FROM test WHERE text = '1234567890123456789'
);

I've found the answer by myself:
List<Param<?>> params = new LinkedList<>();
params.add(DSL.val("1234567890123456789"));
List<Field<?>> fields = new LinkedList<>();
fields.add(TEST.TEXT);
SelectConditionStep<Record1<TEXT>> notExistsSelect = context.select(TEST.TEXT).from(TEST).where(TEST.TEXT.eq("1234567890123456789"));
SelectConditionStep<Record> insertIntoSelect = context.select(params).whereNotExists(notExistsSelect);
context.insertInto(TEST, fields).select(insertIntoSelect).execute();
But it would be great, if we had an ability to do it via InsertQuery. I've not found the way to do it.

How to get fully materialized query from querydsl

I am trying to use querydsl for building dynamic queries for dynamic schemas. I am trying to get just the query instead of having to actually execute it.
So far I have faced two issues:
- The schema.table notation is absent. Instead I only get the table name.
- I have been able to get the query but it separates out the variables and puts '?' instead which is understandable. But I am wondering if there is some way to get fully materialized query including the parameters.
Here is my current attempt and result(I am using MySQLTemplates to create the configuration):
private SQLTemplates templates = new MySQLTemplates();
private Configuration configuration = new Configuration(templates);
String table = "sometable"
Path<Object> userPath = new PathImpl<Object>(Object.class, table);
StringPath usernamePath = Expressions.stringPath(userPath, "username");
NumberPath<Long> idPath = Expressions.numberPath(Long.class, userPath, "id");
SQLQuery sqlQuery = new SQLQuery(connection, configuration)
.from(userPath).where(idPath.eq(1l)).limit(10);
String query = sqlQuery.getSQL(usernamePath).getSQL();
return query;
And what I get is:
select sometable.username
from sometable
where sometable.id = ?
limit ?
What I wanted to get was:
select sometable.username
from someschema.sometable
where sometable.id = ?
limit ?
Update: I came up with this sort of hack to get parameters materialized(Not ideal and would love better solution) But still could not get Schema.Table notation to work:
Hack follows. Please suggest cleaner QueryDsl way of doing it:
String query = cleanQuery(sqlQuery.getSQL(usernamePath));
private String cleanQuery(SQLBindings bindings){
String query = bindings.getSQL();
for (Object binding : bindings.getBindings()) {
query = query.replaceFirst("\\?", binding.toString());
}
return query;
}

To enable schema printing use the following pattern
SQLTemplates templates = MySQLTemplates.builder()
.printSchema()
.build();
SQLTemplates subclasses were used before, but since some time the builder pattern is the official way to customize the templates http://www.querydsl.com/static/querydsl/3.3.1/reference/html/ch02s03.html#d0e904
And to enable direct serialization of literals use
//configuration level
configuration.setUseLiterals(true);
//query level
configuration.setUseLiterals(true);
Here is a full example
// configuration
SQLTemplates templates = MySQLTemplates.builder()
.printSchema()
.build();
Configuration configuration = new Configuration(templates);
// querying
SQLQuery sqlQuery = new SQLQuery(connection, configuration)
.from(userPath).where(idPath.eq(1l)).limit(10);
sqlQuery.setUseLiterals(true);
String query = sqlQuery.getSQL(usernamePath).getSQL();
If you always just want the SQL query string out, move setUseLiterals from query to configuration.
Concerning the usage of Querydsl expressions the usage of code generation like documented here is advised http://www.querydsl.com/static/querydsl/3.3.1/reference/html/ch02s03.html
It will make your code typesafe, compact and readable.
If you want to try Querydsl without code generation you can replace
Path<Object> userPath = new PathImpl<Object>(Object.class, variable);
with
Path<Object> userPath = new RelationalPathBase<Object>(Object.class, variable, schema, table);

When working with QueryDSL, you must provide a template for the database platform to build the query for. I see you are already are doing this here:
private SQLTemplates templates = new MySQLTemplates();
private Configuration configuration = new Configuration(templates);
To make the schema name appear in the generated query, the only way I have found to do this is (there may be an easier way) is to extend the template class and explicitly call this.setPrintSchema(true); inside the constructor. Here is a class that should work for MySql:
import com.mysema.query.sql.MySQLTemplates;
public class NewMySqlTemplates extends MySQLTemplates {
public NewMySqlTemplates() {
super('\\', false);
}
public NewMySqlTemplates(boolean quote) {
super('\\', quote);
}
public NewMySqlTemplates(char escape, boolean quote) {
super(escape, quote);
this.setPrintSchema(true);
}
}
Then simply use this NewMySqlTemplates class in place of the MySQLTemplates class like this:
private SQLTemplates templates = new NewMySQLTemplates();
private Configuration configuration = new Configuration(templates);
I have this working using PostgresTemplates, so I may have a typo or mistake in the NewMySqlTemplates class above, but you should be able to get it to work. Good luck!

new to android - understanding source of content provider

As I was working through the following tutorial, I came across this code :
public void onClickRetrieveStudents(View view) {
// Retrieve student records
String URL = "content://com.example.provider.College/students";
I am interested to see what kind of data this is, so I tried to go to the website http://com.example.provider.College/students to view the data, however it just gave some kind of error. Therefore my question is , is this URL some kind of xml document? what exactly is the format for this data... and how can I view it ?

I would recommend you familiarize yourself with the following documenation:
Content Providers:
http://developer.android.com/guide/topics/providers/content-providers.html
Essentially when you pass that "URL" to the ContentResolver (presumably you're doing somethign like this):
// Queries the user dictionary and returns results
mCursor = getContentResolver().query(
UserDictionary.Words.CONTENT_URI, // The content URI of the words table
mProjection, // The columns to return for each row
mSelectionClause // Selection criteria
mSelectionArgs, // Selection criteria
mSortOrder); // The sort order for the returned rows
You're asking android to resolve that URL to a ContentProvider which is set up to handle that URL. The URL is not "imaginary" so much as it's targets are Local objects and processes which exist and are defined by applications which use the ContentProvider mechanism to store and make data available to other applications.
The goal of that URL (which is converted to a URI in this case) is to specify which ContentProvider you want, and what you want from it.
ContentProviders are generally used by applications that want to manage a database and make that information available to other applications while minimizing access violations etc..
EDIT:
This code is from your tutorial. See added comments:
/// this url points to the content provider.
//The content provider uses it to
///reference a specific database which it has knowledge of
//This URI doesn't represent an
//actual FILE on your system, rather it represents a way for you to tell the content //provider what DATABASE to access and what you want from it.
String URL = "content://com.example.provider.College/students";
// This line converts yoru "URL" into a URI
Uri students = Uri.parse(URL);
/// This call returns a Cursor - a cursor is a object type which contains the results of your QUERY in an order manner. IN this case it is a set of rows, each of which has a number of columns coresponding to your query and database, which can be iterated over to pull information from the DB..
/// managedQuery takes, as an argument, the URI conversion of the URL - this is
// where you are actually calling to the contentprovider, asking it to do a query on the
// databse for some information
Cursor c = managedQuery(students, null, null, null, "name");
// This line moves to the first ROW in the cursor
if (c.moveToFirst()) {
// this does somethign as long as the while loop conditional is true.
do{
// This line creates a pop up toast message with the information stored in the columns of the row you the cursor is currently on.
Toast.makeText(this,
c.getString(c.getColumnIndex(StudentsProvider._ID)) +
", " + c.getString(c.getColumnIndex( StudentsProvider.NAME)) +
", " + c.getString(c.getColumnIndex( StudentsProvider.GRADE)),
Toast.LENGTH_SHORT).show();
} while (c.moveToNext());
}
Your question in the comments was:
"all I need is an example of this file: String URL = "content://com.example.provider.College/students"; , what would the data look like ? "
The answer to this is that you have an Sqlite Database on your phone somewhere - generally (and in this case definitely) created by the application and/or content provider you are accessing. You also know that the content resolver accepts this URI and some other information and will return you a CURSOR.
This question addresses what a cursor is.
use of cursor in android
If you read the tutorial fully you will find this code::
public class StudentsProvider extends ContentProvider {
static final String PROVIDER_NAME = "com.example.provider.College";
static final String URL = "content://" + PROVIDER_NAME + "/students";
static final Uri CONTENT_URI = Uri.parse(URL);
static final String _ID = "_id";
static final String NAME = "name";
static final String GRADE = "grade";
You will also find, in the manifest of your tutorial:
<provider android:name="StudentsProvider"
android:authorities="com.example.provider.College">
</provider>
Which is the registration of your ContentProvider for the URI at question.
You will note that your URL and the "PROVIDER_NAME" and "URL" have eerie similarities. This is because the ContentProvider is utilizing these values to identify itself as the resolver for this partiuclar URI to the android system.
You should create the files as described in the tutorial, make the sample app function, and you will be able to start understanding this more clearly.

It's not real, and it's not a web url. That is an example of a hypothetical ContentURI.
As an example, you might consult the UserDictionary like so -
// Queries the user dictionary and returns results
mCursor = getContentResolver().query(
UserDictionary.Words.CONTENT_URI, // The content URI of the words table
mProjection, // The columns to return for each row
mSelectionClause // Selection criteria
mSelectionArgs, // Selection criteria
mSortOrder); // The sort order for the returned rows
You might also create your own.

Jena TDB to store and query using API

I am new to both Jena-TDB and SPARQL, so it might be a silly question. I am using tdb-0.9.0, on Windows XP.
I am creating the TDB model for my trail_1.rdf file. My understanding here(correct me if I am wrong) is that the following code will read the given rdf file in TDB model and also stores/load (not sure what's the better word) the model in the given directory D:\Project\Store_DB\data1\tdb:
// open TDB dataset
String directory = "D:\\Project\\Store_DB\\data1\\tdb";
Dataset dataset = TDBFactory.createDataset(directory);
Model tdb = dataset.getDefaultModel();
// read the input file
String source = "D:\\Project\\Store_DB\\tmp\\trail_1.rdf";
FileManager.get().readModel( tdb, source);
tdb.close();
dataset.close();
Is this understanding correct?
As per my understanding since now the model is stored at D:\Project\Store_DB\data1\tdb directory, I should be able to run query on it at some later point of time.
So to query the TDB Store at D:\Project\Store_DB\data1\tdb I tried following, but it prints nothing:
String directory = "D:\\Project\\Store_DB\\data1\\tdb" ;
Dataset dataset = TDBFactory.createDataset(directory) ;
Iterator<String> graphNames = dataset.listNames();
while (graphNames.hasNext()) {
String graphName = graphNames.next();
System.out.println(graphName);
}
I also tried this, which also did not print anything:
String directory = "D:\\Project\\Store_DB\\data1\\tdb" ;
Dataset dataset = TDBFactory.createDataset(directory) ;
String sparqlQueryString = "SELECT (count(*) AS ?count) { ?s ?p ?o }" ;
Query query = QueryFactory.create(sparqlQueryString) ;
QueryExecution qexec = QueryExecutionFactory.create(query, dataset) ;
ResultSet results = qexec.execSelect() ;
ResultSetFormatter.out(results) ;
What am I doing incorrect? Is there anything wrong with my understanding that I have mentioned above?

For part (i) of your question, yes, your understanding is correct.
For part (ii), the reason that listNames does not return any results is because you have not put your data into a named graph. In particular,
Model tdb = dataset.getDefaultModel();
means that you are storing data into TDB's default graph, i.e. the graph with no name. If you wish listNames to return something, change that line to:
Model tdb = dataset.getNamedGraph( "graph42" );
or something similar. You will, of course, then need to refer to that graph by name when you query the data.
If your goal is simply to test whether or not you have successfully loaded data into the store, try the command line tools bin/tdbdump (Linux) or bat\tdbdump.bat (Windows).
For part (iii), I tried your code on my system, pointing at one of my TDB images, and it works just as one would expect. So: either the TDB image you're using doesn't have any data in it (test with tdbdump), or the code you actually ran was different to the sample above.

The problem in your part 1 code is, I think, you are not committing the data .
Try with this version of your part 1 code:
String directory = "D:\\Project\\Store_DB\\data1\\tdb";
Dataset dataset = TDBFactory.createDataset(directory);
Model tdb = dataset.getDefaultModel();
// read the input file
String source = "D:\\Project\\Store_DB\\tmp\\trail_1.rdf";
FileManager.get().readModel( tdb, source);
dataset.commit();//INCLUDE THIS STAMEMENT
tdb.close();
dataset.close();
and then try with your part 3 code :) ....