[unculture]

Repository pattern is useful if you really feel like you're going to need to switch out your database layer for something else at some point in the future, but I've literally never seen this happen in my career ever. Otherwise, it's just duplicate code you have to write.

[kelafoja]

What is the alternative that you use, how do you provide data access in a clean, separated, maintainable way?

I have seen it a lot in my career, and have used it a lot. I've never used it in any situation to switch out a database layer for something else. It seems like we have very different careers.

I also don't really see how it duplicates code. At the basic level, it's practically nothing more than putting database access code in one place rather than all over the place.

[BerislavLopac]

OK, let's first define some things.

What we are talking about is a "transformation" or "mapper" layer isolating your domain entities from the persistence. If this is what we call "Repository" then yes, I absolutely agree with you -- this is the right approach to this problem. But if the "Repository pattern" means a complex structure of abstract and concrete classes and inheritance trees -- as I have usually seen it implemented -- then it is usually an overkill and rarely a good idea.

[kelafoja]

Thanks. In my mind, anything about complex structures of (abstract) classes and/or inheritance trees has nothing to do with a Repository pattern.

As I understand it, Repository pattern is basically a generalization of the Data Access Object (DAO) pattern, and sometimes treated synonymously.

The way I mean it and implement it, is basically for each entity have a separate class to provide the database access. E.g. you have a Person (not complex at all, simply a value object) and a PersonRepository to get, update, and delete Person objects.

Then based on the complexity and scope of the project, Person either 1-to-1 maps to a e.g. a database table or stored object/document, or it is a somewhat more complex object in the business domain and the repository could be doing a little bit more work to fetch and construct it (e.g. perhaps some joins or more than 1 query for some data).

[BerislavLopac]

> for each entity have a separate class to provide the database access

Let me correct you: for each entity that needs database access. This is why I'm talking about layers here: sometimes entities are never persisted directly, but only as "parts" or "relations" of other entities; in other cases you might have a very complex persistence implementation (e.g. some entities are stored in a RDB, while others in a filesystem) and there is no clear mapping.

I recommend you to approach this from the perspective of each domain entity individually; "persistability" is essentially just another property which might or might not apply in each case.

[kelafoja]

Naturally, Repository is a pattern for data(base) access, so it should have nothing to do with objects that are not persisted. I used "entity" as meaning a persisted object. That was not very clear, sorry.

[BerislavLopac]

Well, again, that is not completely straightforward - what exactly is a "persisted object"? We have two things here that are usually called entities:

1. The domain entities, which are normally represented as native objects in our codebase. They have no idea whether they need to be persisted and how.

2. The database entities, which are - in RDBs at least - represented by tables.

It is not uncommon that our entities of the first type can easily be mapped 1:1 to our entities of the second type - but that is far from guaranteed. Even if this is the case, the entities will be different because of the differences between the two "worlds": for example, Python's integer type doesn't have a direct equivalent in, say, PostgreSQL (it has to be converted into smallint, integer, bigint or numeric).

In my "correction" above I was talking about the domain entities, and my phrasing that they "need database access" is not fully correct; it should have been "need to be persisted", to be pedantic.

[lazyasciiart]

I’ve seen it, but of course there was no strict enforcement of the pattern so it was a nightmare of leakage and the change got stuck half implemented, with two databases in use.