test.scala

case class GHUser(
                 login: String,
                 id: Long,
                 avatar_url: String,
                 gravatar_id: Option[String],
                 url: String,
                 html_url: String,
                 followers_url: String,
                 following_url: Option[String],
                 gists_url: Option[String],
                 starred_url: String,
                 subscriptions_url: String,
                 organizations_url: String,
                 repos_url: String,
                 events_url: Option[String],
                 received_events_url: String,
                 `type`: String,
                 site_admin: Boolean
                 )
case class GHPullRequestMini(project :String,
                             url: String,
                             title: String,
                             state: String,
                             user: GHUser,
                             body: String,
                             created_at: String,
                             updated_at: String,
                             closed_at: Option[String],
                             merged_at: Option[String],
                             assignee: Option[GHUser],
                             branch: String
                              )
implicit val GHUserJsonWrite = {
    import play.api.data.mapping.json.Writes._ // let's no leak implicits everywhere
    Write.gen[GHUser, JsObject]
  }
implicit val GHPullRequestMiniJsonWrite = {
    import play.api.data.mapping.json.Writes._ // let's no leak implicits everywhere
    Write.gen[GHPullRequestMini, JsObject]
  }
  
   val pullRequests:Seq[GHPullRequestMini] = ...
   val json = To[Seq[GHPullRequestMini], JsObject](pullRequests)
   // => could not find implicit value for parameter w: play.api.data.mapping.WriteLike[Seq[models.GHPullRequestMini],play.api.libs.json.JsObject]

Mouai en fait pour generer le Write il faut un JsObject car tu as besoin du Monoid sur ce type. Par contre pour la Seq i tu faut une JsValue du coup il faut que tu type explicitement GHPullRequestMiniJsonWrite

Exemple:

case class Foo(s: String, i: Int)
val foos = Seq(Foo("foo", 1))
implicit val ws: Write[Foo, JsValue] = Write.gen[Foo, JsObject]
scala> To[Seq[Foo], JsValue](foos)
res7: play.api.libs.json.JsValue = [{"s":"foo","i":1}]