Persisting objects in SugarORM

问题

I have a Book class:

public class Book extends SugarRecord {
    private String mBookName;
    private String mAuthorName;
    private List<Page> mPageList;

    public Book() {

    }

    public Book(String bookname, String authorName) {
        mBookName = bookname;
        mAuthorName = authorName;
        mPageList = new ArrayList<>();
    }

    public String getAuthorName() {
        return mAuthorName;
    }

    public void setAuthorName(String authorName) {
        mAuthorName = authorName;
    }

    public String getBookName() {
        return mBookName;
    }

    public void setBookName(String bookName) {
        mBookName = bookName;
    }

    public void addPage(Page page) {
        mPageList.add(page);
    }

}

and the Page class:

public class Page extends SugarRecord {
    private String mText;

    public Page() {

    }
    public Page(String text) {
        mText = text;
    }

    public String getText() {
        return mText;
    }

    public void setText(String text) {
        mText = text;
    }
}

I am testing it with this:

public class MainActivity extends AppCompatActivity {

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        Book book = new Book("Some Book Title", "John Doe");

        Page page1 = new Page("Once upon a time there was a very lonely bunny who wanted some friends.");
        Page page2 = new Page("So he found some friends, and everyone was happy.");
        Page page3 = new Page("The end!");

        book.addPage(page1);
        book.addPage(page2);
        book.addPage(page3);

        book.save();
    }
}

However it is not working as expected. It is trying to make mPageList its own column with this .schema:

CREATE TABLE BOOK ( ID INTEGER PRIMARY KEY AUTOINCREMENT , M_AUTHOR_NAME TEXT, M_BOOK_NAME TEXT, M_PAGE_LIST  );

What I'd really like it to do is not treat the list as its own column but instead save the Pages to the PAGE table, with additional ids that reference this Book class (so what I am expecting is something like ID, BOOK_ID, M_TEXT). In short, persistence operations that cascade through nested child objects.

Can this be done in SugarORM?

回答1:

No ORM database(SugarORm, DBFLow etc) supports List column. As you know sql don't have this datatype as well.

That's the reason why you are getting this error. If you ask me how you are saving list to ORM. I use Gson.

Declare Pagelist as string.

String Pagelist;

Before saving it to database convert it to Json string with the help Gson library.

Gson gson = new Gson();
String value = gson.toJson(your_page_list);

when retrieving from database convert the json string to List using Gson.

List<Page> page_list;
  Type typeIndicatorForGson = new TypeToken<ArrayList<Page>>() {}.getType(); 
 Gson myGson = new Gson();  
 page_list = myGson.fromJson(page_json_data_from_database, typeIndicatorForGson);

回答2:

No List<Object> available on SugarORM. The way you can manage this is a little tricky. In few words, you can manage 1 to N relations, upside down. Take a look to the next example Lets suppose a Team object which can have N Person objects. Normally you will use a List<Person> in your class Team in this way:

public class Team {
   String teamName;
   List<Person>
   ...
}

public class Person {
   String name;
   String rol;
   ...
}

Well, it is not possible on SugarORM. But you can add Team as a property in Person, so, any Person's instance should contain a reference to the Team object it belong.

public class Team extends SugarRecord<Team> {
   String teamName;
   ...
}

public class Person extends SugarRecord<Person> {
   String name;
   String rol;
   Team team;
   ...
}

Then you can get all the Person objects from Team with a method (in the Team class) like:

public class Team extends SugarRecord<Team> {
   String teamName;
   ...

    public List<Person> getPersons(){
            return Person.find(Person.class, "id = ?", String.valueOf(this.getId()));
    }
}

So, you can manage 1 to N relations, but you can't manage N to M relationships (Person belonging to more than one Team object). IMO the way to manage this is using an Associative Entity in order to split N to M into two 1 to N relationships. As you can see SugarORM is not allowing you to think just in terms of objects, but, any case you can save a lot of boiler plate coding.

来源：https://stackoverflow.com/questions/38975130/persisting-objects-in-sugarorm

标签

java

android

sqlite

persistence

sugarorm