followers initial commit

backend/src/main/resources/db/migration/V3__Create_followers_table.sql

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+CREATE TABLE followers (
+    follower_id TEXT NOT NULL,
+    followed_id TEXT NOT NULL,
+    CONSTRAINT followers_pk PRIMARY KEY(follower_id, followed_id)
+);

backend/src/main/scala/twitterclone/repositories/domain/FollowRepository.scala

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+package twitterclone.repositories.domain
+
+import twitterclone.model.Id
+import twitterclone.model.user.User
+
+trait FollowRepository[F[_]] {
+
+  def addFollower(followerId: Id[User], followeeId: Id[User]): F[Unit]
+
+  def removeFollower(followerId: Id[User], followeeId: Id[User]): F[Unit]
+
+  def getFollowers(userId: Id[User]): F[Set[Id[User]]]
+
+  def follows(followerId : Id[User], followeeId: Id[User]) : F[Boolean]
+}

backend/src/main/scala/twitterclone/repositories/interpreters/local/LocalFollowRepository.scala

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+package twitterclone.repositories.interpreters.local
+
+import cats.Applicative
+import twitterclone.model.Id
+import twitterclone.model.user.User
+import twitterclone.repositories.domain.FollowRepository
+import cats.implicits._
+
+import scala.collection.mutable
+
+object LocalFollowRepository {
+
+  case class DirectedFollowRelationship(follower: Id[User], followed: Id[User])
+
+  def apply[F[_] : Applicative](state : mutable.Set[DirectedFollowRelationship] = mutable.Set.empty) = new FollowRepository[F] {
+    override def addFollower(followerId: Id[User], followeeId: Id[User]): F[Unit] = (state += DirectedFollowRelationship(followerId, followeeId)).pure[F].void
+
+    override def removeFollower(followerId: Id[User], followeeId: Id[User]): F[Unit] = ((state += DirectedFollowRelationship(followerId, followeeId)).pure[F].void)
+
+    override def getFollowers(userId: Id[User]): F[Set[Id[User]]] = (state.filter(_.followed == userId).map(_.follower).toSet.pure[F])
+
+    override def follows(followerId: Id[User], followeeId: Id[User]): F[Boolean] = state.contains(DirectedFollowRelationship(followerId, followeeId)).pure[F]
+  }
+
+}

backend/src/main/scala/twitterclone/repositories/interpreters/postgres/PostgresFollowRepository.scala

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+package twitterclone.repositories.interpreters.postgres
+
+object PostgresFollowRepository {
+
+}