I'm new to greenDAO and I'm working on writing the DaoGenerator. One issue that I've run into is that I have a user table and a wallpost table. I would like to be able to have two columns in the wallpost table that are toMany relations to the user table (the wall owner and the posting user) they may or may not be the same user, but so far it doesn't look like it is possible to have two toMany relations that point to a single table in the same table.
Is there a better way to do this/a way to make this possible? I'm hoping to be able to load the wall posts and fetch the wall owner and posting user by calling .getOwner() and .getPoster().
Thanks
You must set names for the relations. Have a look at the (just improved) section called Relation Names and multiple Relations of the documentation on relations. It comes with an example:
Property pictureIdProperty = user.addLongProperty("pictureId").getProperty();
Property thumbnailIdProperty = user.addLongProperty("thumbnailId").getProperty();
user.addToOne(picture, pictureIdProperty);
user.addToOne(picture, thumbnailIdProperty, "thumbnail");
Related
I have to join three tables in Firebase to fetching the value from it.
consider three table A,B, C.Table A contains attribute as(id1,name).Table B contains attribute as(id2,Bname).C contains attribute as(id1,id2).the relation between table A and table B is M to M.
I want this query to executed in Firebase.
the query is
select A.id1 from A,B,C where A.id1=C.id1 and B.id2=C.id2 and B.Bname="?"
Thank You!.
The Firebase Realtime database is not like an sql database, it has no idea of joins etc. References are fairly cheap, so maybe create 3 references to the 3 locations that contain the data and run these 3 "queries" and tie the results together to get the final view of the data you want. Something like RxJava might help in running the 3 "queries" and chaining the results together.
Try joining the Firebase Slack channel, it has a lot of good posts on how to structure your Firebase data etc.
https://firebase.community/
I'm implementing a Content Provider, which is backed by a fairly complex SQLite DB schema. The database has a few junction tables and I'm unsure whether they should be visible to the user of the Content Provider or not.
The parent tables are exposed via the Contract, each one has its own content URI, etc. Now, when inserting data via ContentResolver#applyBatch() method, I create ContentProviderOperation per each table's content URI. So far everything is clear. But my problem is, how should the junction tables be populated, as they don't have their own content URIs?
To illustrate this, here's an example. I have 2 "parent" tables, Movies and Actors. The relationship between them is many-to-many and therefore I have a junction table called MoviesActors.
To insert at one batch I do the following:
List<ContentProviderOperation> operations = new ArrayList<>;
// movie
operations.add(ContentProviderOperation.newInsert(Contract.Movie.ContentUri).withValue("movie_id", "23asd2kwe0231123sa").build());
// actor
operations.add(ContentProviderOperation.newInsert(Contract.Actor.ContentUri).withValue("actor_id", "89asd02kjlwe081231a").build());
getContentResolver().applyBatch(authority, operations);
The junction table MoviesActors should be inserted with a row containing movie_id and actor_id. How do I take care of the junction table in this situation?
The only thing, which comes to my mind is extend the Contract to have content URI pointing to the junction tables and add another ContentProviderOperation, since otherwise, how do you communicate movie_id and actor_id to ContentProvider#applyBatch()?
I rather not expose the junction table to the user of the ContentProvider, but I might be wrong here... perhaps that's how it should be done on Android?
I've searched this topic for days already and haven't found an answer to that.
Any help would be greatly appreciated.
Bonus question:
Is it necessary to expose every single table via the Contract? For instance, when having child tables in one-to-many relationship. I'm specifically referring to Insert/Update/Delete since I know with Query I can simply do a join, but maybe I'm wrong also here.
Thanks a lot!
NOTE: I'm not interested in 3rd party library solutions.
I think you're tackling the problem from the wrong end. You're trying to design an interface to match your database structure, but the interface should come first.
In the first place, the interface should meet all the requirements of your ContentProvider client. If your ContentProvider client needs access to the junction table you'll have to expose it (in some way, see below), otherwise you don't have to. A good interface hides the actual implementation details, so the ContentProvider client doesn't need to care about whether the ContentProvider is backed by an SQLite database, by a bunch of in-memory maps or even a web-service.
Also, you should not think of a ContentProvider just as an interface to a database and the Contract as the database schema. A ContentProvider is much more versatile and powerful than that. The major difference is that ContentProviders are addressed by URIs whereas in SQL you just have table names. In contrast to a table name, a URI has a structure. URIs have a path that identifies the object (or directory of objects) that you want to operate on. Also you can add query parameters to a URI to modify the behavior of an operation. In this respect a ContentProvider can be designed much like a RESTful service.
See below for a concrete (but incomplete) example of a Contract of a simple movie database. This is basically how one would design a RESTful web-service, except for one thing: Just like in your code, movie-id and actor-id are provided by the caller. A real RESTful service would create and assign these automatically and return them to the caller. A ContentProvider can only return long IDs when inserting new objects.
Insert a new movie
insert on /movies/
Values: {"movie_id": <movie-id>, "title": <movie-title>, "year": ...}
Insert a new actor
insert on /actors/
Values: {"actor_id": <actor-id>, "name": <actor-name>, "gender": ...}
Add an existing actor to a movie
insert on /movies/movie-id/actors/
Values: {"actor_id": <actor-id>}
Add an existing movie to an actor:
insert on /actors/actor-id/movies/
Values: {"movie_id": <movie-id>}
Optional: add a new actor directly to a movie:
insert on /movies/movie-id/actors/
Values: {"actor_id": <actor-id>, "name": <actor-name>, "gender": ... }
If no actor with the given id exists, this operation will create the new actor and link it to the movie in a single step. If an actor with this ID already exists an exception would be thrown.
The same could be done the other way round, adding a new movie to an actor.
Delete an actor from a movie
delete on /movies/movie-id/actors/actor-id
or
delete on /actors/actors-id/movies/movie-id
Get all movies
query on /movies/
Get a specific movie
query on /movies/movie-id
Get all actors playing in a specific movie
query on /movies/movie-id/actors/
Get all movies a specific actor has played in
query on /actors/actor-id/movies/
The optional query selection statement can be used to filter the result. To get movies from the last 10 years a specific actor has played in, you would add the selection movies_year>=2005 to the last query.
By using a contract like this you wouldn't expose the junction table, instead you provide a REST-like interface to your database.
The job of the ContentProvider is to map these operations onto the database or any other back-end.
I put my data in 3 tables(Links, Images and PDF)
each table has columns(university, faculty, grade, and description,...)
I want to retrieve description column in the 3 tables.
where university, faculty, and grade equal to certain values.
and sort them with creation date.
how can I perform that query in parse?
I'm not familiar with Android, but I'm pretty sure Parse does not support "Join" in the way a SQL database does. You could nest the queries, performing the next one in the previous one's completion block.
However, if you regularly want to get data from those 3 tables, I'd suggest you make it 1 table instead, with a column "Content" instead of Link/Img/PDF. Images and PDFs would probably be stored as PFFiles anyway, and you can put link as either its own string column or putting it in a file. You could also add a column "type" if you want to be able to query a specific type, or just keep track of which columns contains which data.
Then you could query the "Content" class, on the keys you want.
I think this link might help you
https://parse.com/docs/js/guide#relations and it is quite simple and nicely explained . You can't do it directly in the database, though.
I am using Parse for one of my android application and struggling for a query to get output.
Application Parse DB has three table which is as described below-
User: user related information with unique object Id
Post: Contains Post related information with unique object Id and UserId(Pointer to user table) who create the post
Like: UserId(Pointer to user table) represent who is Liking the post, PostId(Pointer to post table) which post is liking and unique
object Id
Now I want to make a single compound query for All post for a specific user along-with total number of likes to corresponding post. Please suggest a solution/query for that
When a user creates a post, assign the user objectID to some post identifier property in the post table.
To retrieve all posts from a user, a query like this may help:
query.whereEqualTo("objectID", currentUser.objectID)
If you're just concerned about the like count, rather than each user who has liked the post, you may want to have an Integer value in the post table that is incremented after a user presses the like button on that post.
You should create your own webHook.
Following this exemples here you can easily learn and develop your own stuff, you can even add some triggers to your application!
https://parse.com/docs/cloudcode/guide#cloud-code-cloud-functions
https://parse.com/docs/cloudcode/guide#cloud-code-beforesave-triggers
If User table doesn't have Post<.Relation> column
I suggest you to read about how to join queries.
f.g. make a query1 to get the user, then another query2 to get all posts with whereMatchesQuery('UserColumnInPostTable', query1) function.
Now, you are going to get all posts of a specific user efficiently.
.
If User table have Post<.Relation> column
this is the easiest way
Just get the user object and get the relation object, then get the query, and find in background
this is more of a question of theory than anything else. I am writing an android app that uses a pre-packaged database. The purpose of the app is solely to search through this database and return values. Ill provide some abstract examples to illustrate my implementation and quandary. The user can search by: "Thing Name," and what I want returned to the user is values a, b, and c. I initially designed the database to have it all contained on a single sheet, and have column 1 be key_index, column 2 be name, column 3 be a, etc etc. When the user searches, the cursor will return the key_index, and then use that to pull values a b and c.
However, in my database "Thing alpha" can have a value a = 4 or a = 6. I do not want to repeat data in the database, i.e. have multiple rows with the same thing alpha, only separate "a" values. So what is the best way to organize the data given this situation? Do I keep all the "Thing Names" in a single sheet, and all the data separately. This is really a question of proper database design, which is definitely something foreign to me. Thanks for your help!
There's a thing called database normalization http://en.wikipedia.org/wiki/Database_normalization. You usually want to avoid redundancy and dependency in the DB entities using a corresponding design with surrogate keys and foreign keys and so on. Your "thing aplpha" looks like you want to have a many-to-many table like e.g. one or many songs belong/s to the same or different genres. You may want to create dictionary tables to hold your id,name pairs and have foreign keys referencing these tables. In your case it will be mostly a read-only DB so you might want to consider creating indexes with high FILLFACTOR percentage don't think sqlite allows it to do though. There're many ways to design the database. Everything depends on the purpose of DB. You can start with a design of your hardware like raids/file systems/db block sizes to match the F-System's block sizes in order to keep the I/O optimal and where to put your tablespaces/filegroups/indexes to balance the i/o load. The whole DB design theory/task is really a deep subject which is not to be underestimated nor is a matter of few sentences in the answer of stackoverflow. :)
without understanding your data better here is my guess at what you are looking for.
table: product
- _id
- name
table: attribute
- product_id
- a