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How to get an entity-related object correctly

I have approximately the following entity:
public class Article {
private String name;
private Long fileId;
}
As you can see, it has a field fileld that contains the id of the associated file, which is also an entity. However, the file does not know anything about the Article, so the only thing that connects them is the fileId field in the Article. Therefore, they must be explicitly linked so as not to get lost. Now to get a linked file, I have to make a separate query to the database for each Article. That is, if I want to get a list of 10 Articles, I need to make a request to the database 10 times and get the file by its id. This looks very inefficient. How can this be done better? I use jooq, so I can't use JPA, so I can't substitute a file object instead of the fileId field. Any ideas?

I'm going to make an assumption that your underlying tables are something like this:
create table file (
id bigint primary key
content blob
);
create table article (
name text,
file_id bigint references file
);
In case of which you can fetch all 10 files into memory using a single query like this:
Result<?> result =
ctx.select()
.from(ARTICLE)
.join(FILE).on(ARTICLE.FILE_ID.eq(FILE.ID))
.fetch();

Failed to make bulk upsert using mongo

I'm trying to do upsert using mongodb driver, here is a code:
BulkWriteOperation builder = coll.initializeUnorderedBulkOperation();
DBObject toDBObject;
for (T entity : entities) {
toDBObject = morphia.toDBObject(entity);
builder.find(toDBObject).upsert().replaceOne(toDBObject);
}
BulkWriteResult result = builder.execute();
where "entity" is morphia object. When I'm running the code first time (there are no entities in the DB, so all of the queries should be insert) it works fine and I see the entities in the database with generated _id field. Second run I'm changing some fields and trying to save changed entities and then I receive the folowing error from mongo:
E11000 duplicate key error collection: statistics.counters index: _id_ dup key: { : ObjectId('56adfbf43d801b870e63be29') }
what I forgot to configure in my example?

I don't know the structure of dbObject, but that bulk Upsert needs a valid query in order to work.
Let's say, for example, that you have a unique (_id) property called "id". A valid query would look like:
builder.find({id: toDBObject.id}).upsert().replaceOne(toDBObject);
This way, the engine can (a) find an object to update and then (b) update it (or, insert if the object wasn't found). Of course, you need the Java syntax for find, but same rule applies: make sure your .find will find something, then do an update.
I believe (just a guess) that the way it's written now will find "all" docs and try to update the first one ... but the behavior you are describing suggests it's finding "no doc" and attempting an insert.

GAE JAVA - Getting Object by Property

I have Entity say, User { id (primary_key), phone } to be stored in Datastore.
While Retrieving same i can use getObjectById(User.class,id) to get object. Is there way to get object with non-key property, say phone.
As per the documentation, Datastore creates index updates for Property "phone" too.
How do we use this index to get result?

You can simply use a JDO Query like following and Datastore will query on the non-key phone property (assuming you haven't set to to be unindexed).
q = pm.newQuery(User.class,"phone == '1234567890'");

Proper way to insert record with unique attribute

I am using spring, hibernate and postgreSQL.
Let's say I have a table looking like this:
CREATE TABLE test
(
id integer NOT NULL
name character(10)
CONSTRAINT test_unique UNIQUE (id)
)
So always when I am inserting record the attribute id should be unique
I would like to know what is better way to insert new record (in my spring java app):
1) Check if record with given id exists and if it doesn't insert record, something like this:
if(testDao.find(id) == null) {
Test test = new Test(Integer id, String name);
testeDao.create(test);
}
2) Call straight create method and wait if it will throw DataAccessException...
Test test = new Test(Integer id, String name);
try{
testeDao.create(test);
}
catch(DataAccessException e){
System.out.println("Error inserting record");
}
I consider the 1st way appropriate but it means more processing for DB. What is your opinion?
Thank you in advance for any advice.

Option (2) is subject to a race condition, where a concurrent session could create the record between checking for it and inserting it. This window is longer than you might expect because the record might be already inserted by another transaction, but not yet committed.
Option (1) is better, but will result in a lot of noise in the PostgreSQL error logs.
The best way is to use PostgreSQL 9.5's INSERT ... ON CONFLICT ... support to do a reliable, race-condition-free insert-if-not-exists operation.
On older versions you can use a loop in plpgsql.
Both those options require use of native queries, of course.

Depends on the source of your ID. If you generate it yourself you can assert uniqueness and rely on catching an exception, e.g. http://docs.oracle.com/javase/1.5.0/docs/api/java/util/UUID.html
Another way would be to let Postgres generate the ID using the SERIAL data type
http://www.postgresql.org/docs/8.1/interactive/datatype.html#DATATYPE-SERIAL
If you have to take over from an untrusted source, do the prior check.

Overwrite Datastore entities in Google App Engine- Java

I have an application in which I want to overwrite an individual entity. This is how I originally create entity logs:
Entity log = new Entity("Log", "Logkey");
String property1 = req.getParameter("property1");
String property2 = req.getParameter("property2");
log.setProperty("property1", property1);
log.setProperty("property2", property2);
datastore.put(log);
Here is how entity logs are retrieved to be overwritten:
Query query = new Query("Log", "Logkey")
.setFilter(timeStampFilter);
List<Entity> logs = datastore.prepare(query).asList(FetchOptions.Builder.withLimit(1));
request.setAttribute("logs", logs);
and sent to a jsp form page as value="${log.properties.property1}" where they should be overwritten. This entry is then sent to a second servlet with the POST method and requested as parameters as in the earlier code but saved as a new entity with the same kind:
Entity edit_log = new Entity("Log", "Logkey");
String property1 = req.getParameter("property1");
String property2 = req.getParameter("property2");
edit_log.setProperty("property1", property1);

For rewriting and existing entity, after retreiving a specific log by timestamp, you can get the key of this log using getKey() method and then create an entity with this key and the new details. Now when you put this new entity to the datastore it will replace the earlier one with the same key

With the code you've written, you only have a single Log entity in your datastore with the key "Logkey" that you are constantly overwriting.
If you're using some other code to retrieve entities and rewrite them, then you'll need to show that other code. Otherwise, this question is poorly written, because the code given is already describing what you want to do (always overwrite the same entity).
If you have code elsewhere creating/saving entities, it's best to show that too.
Edit: It looks like you end up creating a nested entity with the data from the old entity in a new entity with the same key. It's far easier just to reuse the entity you received from the query.
log = logs.get(0)
log.setProperty("property1", req.getParameter("property1");
log.setProperty("property2", req.getParameter("property2");
datastore.put(log);
Furthermore, since you actually know the key ("Logkey"), you don't need to issue a datastore query, you can just fetch the entity by key - which is good because you get around eventual-consistency behavior.

If your new entity has the same key as your original one, then when you store it it will override the old entity.