combining multiple pipes

[alexmnyc]

What is the best way to construct a query with dynamically added pipes similar to

var query =  User.where(_.id eqs someId)

if (something) query = query.modify(_.likes push List("like"))

if (somethingElse) query= query.modify(_.dislikes push List("disklike"))

query.updateMulti()

[Jorge Ortiz]

At Foursquare, we have some helper methods/implicits for this situation. With the following code:

    sealed class Identity[A](protected val _value: A) {

      def applyIf[B >: A](pred: Boolean, f: A => B): B = if (pred) f(_value) else _value

      def applyOpt[B](opt: Option[B])(f: (A, B) => A): A = opt.map(b => f(_value, b)).getOrElse(_value)

    }

    object Identity {

      implicit def wrapIdentity[A](anything: A): Identity[A] = new Identity(anything)

    }

Then, as long as the wrapIdentity implicit is in scope, you can write code like:

    val query = (User

      .where(_.id eqs someId)

      .applyIf(something, _.modify(_.likes push List("like"))))

      .applyIf(somethingElse, _.modify(_.dislikes push List("dislike"))))

    query.updateMulti()

The inferred type usually does the Right Thing. Sometimes you can get yourself in a pickle with the phantom types, but there's usually a way out of it.

Usage of the similar applyOpt method is left as an exercise.

--j

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[Jorge Ortiz]

Open mouth, insert foot. This particular code will get you into a pickle with the types.

You can use the .noop() method to turn any query into a modify query. So what you really want is more like:

    val query = (User

      .where(_.id eqs someId)

      .noop()

      .applyIf(something, _.modify(_.likes push List("like"))))

      .applyIf(somethingElse, _.modify(_.dislikes push List("dislike"))))

    query.updateMulti()