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Some of the image collection s responsibilities include managing the list of tags and keeping track of which tag belongs to which image. Let s start by having a look at the getTags method. Its role is to return a list of all available tags. The method s source code can be seen in Listing 13-18. Because you use the default connection, you create a query, prepare it, and execute it. The query itself contains a DISTINCT clause since the same tag can occur several times for different images. This ensures that you do not get a list with duplicates. When the query has been executed, the results are put in a QStringList that is returned. Listing 13-18. Querying for a list of tags, packaging them in a QStringList, and returning QStringList ImageCollection::getTags() { QSqlQuery qry;
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qry.prepare( "SELECT DISTINCT tag FROM tags" ); if( !qry.exec() ) qFatal( "Failed to get tags" ); QStringList result; while( qry.next() ) result << qry.value(0).toString(); return result; } The other tag management method, the addTag method (see Listing 13-19), adds a tag to a given image. Which image the tag belongs to is specified using an id value. The method does not check for duplicates because the getTags method filters them away, so it is possible to add the same tag several times to the same image. Listing 13-19. Adding a new tag to an image void ImageCollection::addTag( int id, QString tag ) { QSqlQuery qry; qry.prepare( "INSERT INTO tags (id, tag) VALUES (:id, :tag)" ); qry.bindValue( ":id", id ); qry.bindValue( ":tag", tag ); if( !qry.exec() ) qFatal( "Failed to add tag" ); }
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The getIds method deals with images from a tag point of view. It takes a QStringList of tags and returns a list of id values for the images that have at least one of the tags. If no tags are given to the method, it returns all image id values. This is why there are two different queries prepared in the source code shown in Listing 13-20. In the SQL statement handling one or more tags, the IN clause is used. Writing x IN (1, 2, 3) is equal to writing x=1 OR x=2 or x=3. Because the user interface ensures that the tags consist of only the letters a z, you can safely join them together and use them directly in the SQL query.
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Caution You should always try to avoid inserting strings manually into SQL statements; use bindValue
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The SQL statement is ended by a GROUP BY clause, ensuring that you do not get more than one id. The results from the query are put together in a list of integers that is returned. Listing 13-20. Getting every id for a given set of tags (or every id if no tags are given) QList<int> ImageCollection::getIds( QStringList tags ) { QSqlQuery qry; if( tags.count() == 0 ) qry.prepare( "SELECT images.id FROM images" ); else qry.prepare( "SELECT id FROM tags WHERE tag IN ('" + tags.join("','") + "') GROUP BY id" ); if( !qry.exec() ) qFatal( "Failed to get IDs" ); QList<int> result; while( qry.next() ) result << qry.value(0).toInt(); return result; }
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Storing Images in the Database Storing images in a database is not a straightforward task because there are no data types for storing graphics. Instead you have to rely on the BLOB type, which is a binary large object (in plain English: a chunk of raw data). The process of getting a QImage object into a blob can be broken down into three steps. First you create a buffer in memory and save the image to that buffer. The buffer is then converted to a QByteArray, which is bound to a variable in an SQL INSERT query. That query is then executed. This is all done in the addImage method shown in Listing 13-21. As you can see from the highlighted lines, a QBuffer object is created. The image is written to the buffer as a PNG with a QImageWriter. When the buffer contains the image data, you use the data from the buffer in a bindValue call when you prepare the INSERT query to put the image in the database. Looking at the rest of the code, you query the database for the number of images to be able to determine a new id. This method doesn t work if you let the user remove images from the database. It is possible to let the database assign a new id automatically using AUTOINCREMENT when creating the table. That would have solved the problem. But since you support only adding new images, i.e., not removing them, and it is assumed that only one client application is using the database at a time, this solution works.
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