How to construct query in querydsl without domain

问题:

While looking for java libraries to build queries in a database agnostic way I came across many including iciql, querydsl, jooq, joist, hibernate etc.

I wanted something that does not require configuration files and can work with dynamic schemas. For my application, I come to know about the database and the schema at runtime so I won't have any configuration files or domain classes for the schema.

This seems to be one of the core goals of querydsl but going through the documentation for querydsl I see a lot of examples for building dynamic queries using domain classes but I have not come across anything that explains how to build such database agnostic queries using just the dynamic information I have about the schema.

Jooq offers such functionality(See: http://www.jooq.org/doc/3.2/manual/getting-started/use-cases/jooq-as-a-standalone-sql-builder/) but have a restrictive license if I want to expand my focus to Oracle or MS SQL(Which I may not love but need to support).

Can someone with experience in querydsl let me know if such a thing is possible with querydsl, and if yes, how.

If someone know of any other too which can satisfy my requirements, it would be really appreciated.

回答1:

A very simple SQL query such as this:

@Transactional
public User findById(Long id) {
    return new SQLQuery(getConnection(), getConfiguration())
      .from(user)
      .where(user.id.eq(id))
      .singleResult(user);
}

...can be created dynamically like this (without any sugar added):

@Transactional
public User findById(Long id) {
    Path<Object> userPath = new PathImpl<Object>(Object.class, "user");
    NumberPath<Long> idPath = Expressions.numberPath(Long.class, userPath, "id");
    StringPath usernamePath = Expressions.stringPath(userPath, "username");
    Tuple tuple = new SQLQuery(getConnection(), getConfiguration())
      .from(userPath)
      .where(idPath.eq(id))
      .singleResult(idPath, usernamePath);
    return new User(tuple.get(idPath), tuple.get(usernamePath));
}

回答2:

Here is a small variation of ponzao's solution using PathBuilder

@Transactional
public User findById(Long id) {        
    PathBuilder<Object> userPath = new PathBuilder<Object>(Object.class, "user");
    NumberPath<Long> idPath = userPath.getNumber("id", Long.class);
    StringPath usernamePath = userPath.getString("username");
    Tuple tuple = new SQLQuery(getConnection(), getConfiguration())
      .from(userPath)
      .where(idPath.eq(id))
      .singleResult(idPath, usernamePath);
    return new User(tuple.get(idPath), tuple.get(usernamePath));
}

回答3:

Update: Timo has invalidated my original response by showing me how to do what I want WITHOUT having to replace the SQLQuery class. Here is his comment:

query.getSQL(field1, field2, ... fieldN), getSQL is consistent with the
other methods which also take the projection arguments at last

I have removed my unnecessary class. Here is an example of using SQLQuery directly to get the SQL string WITHOUT executing the query (e.g., without using the list method):

SQLQuery rquery = new SQLQuery(connection , dialect);

// Use getSQL with projections
rquery.from(qtable)
    .where(qtable.qfield1.eq("somevalue"));

SQLBindings bindings = rquery.getSQL(qtable.qfield1, qtable.qfield2);

// Get the SQL string from the SQLBindings
System.out.println(bindings.getSql());

// Get the SQL parameters from the SQLBindings for the parameterized query
System.out.println(bindings.getBindings());

This response answers how to use QueryDSL to build out a complete SQL query without actually executing the query. It does not address your additional requirements about "dynamic schemas" and "without domain objects"...