Yes, this is possible with Prisma!. For making this work, you need to specify on your "schema.prisma" file how are models related with each other. That way, code generation will set the possible queries/operations.

Change it to this:

model Post {
  id              Int      @id @unique @default(autoincrement()) @map("id")
  userId          Int      @map("user_id")
  movieId         Int      @unique @map("movie_id")
  title           String   @map("title") @db.Text
  description     String?  @map("description") @db.Text
  tags            Json?    @map("tags")
  createdAt       DateTime @default(now()) @map("created_at") @db.DateTime(0)
  image           String?  @default("https://picsum.photos/400/600/?blur=10") @map("image") @db.VarChar(256)
  year            Int      @map("year")
  submittedBy     String   @map("submitted_by") @db.Text
  tmdbRating      Decimal? @default(0.0) @map("tmdb_rating") @db.Decimal(3, 1)
  tmdbRatingCount Int?     @default(0) @map("tmdb_rating_count")
  ratings         Rating[]

  @@map("posts")
}

model Rating {
  id        Int       @unique @default(autoincrement()) @map("id") @db.UnsignedInt
  userId    Int       @map("user_id") @db.UnsignedInt
  rating    Int       @default(0) @map("rating") @db.UnsignedTinyInt
  entryId   Int
  entry     Post      @relation(fields: [entryId], references: [id])
  createdAt DateTime  @default(now()) @map("created_a") @db.DateTime(0)
  updatedAt DateTime? @map("updated_a") @db.DateTime(0)

  @@id([entryId, userId])
  @@map("ratings")
}

Note: Please follow the naming conventions (singular form, PascalCase). I made those changes for you at the schema above. @@map allows you to set the name you use on your db tables.

Then, after generating the client, you will get access to the relational operations.

    // All posts with ratings data
    const postsWithRatings = await prisma.post.findMany({
        include: {
            // Here you can keep including data from other models
            ratings: true
        },
        // you can also "select" specific properties
    });

    // Calculate on your API
    const ratedPosts = postsWithRatings.map( post => {
        const ratingsCount = post.ratings.length;
        const ratingsTotal = post.ratings.reduce((acc, b) => acc + b.rating, 0)
        return {
            ...post,
            userRating: ratingsTotal / ratingsCount
        }
    })

    // OR...


    // Get avg from db
    const averages = await prisma.rating.groupBy({
        by: ["entryId"],
        _avg: {
            rating: true
        },
        orderBy: {
            entryId: "desc"
        }
    })
    //  Get just posts
    const posts = await prisma.post.findMany({
        orderBy: {
            id: "desc"
        }
    });
    // then match the ratings with posts
    const mappedRatings = posts.map( (post, idx) => {
        return {
            ...post,
            userRating: averages[idx]._avg.rating
        }
    })

You could also create a class with a method for making this easier. But I strongly recommend you to implement GraphQL on your API. That way, you can add a virtual field inside your post type. Any time a post is requested alone or in a list, the average will be calculated. In that same way, you would have the flexibility to request data from other models and the "JOINS" will get handled for you automatically.

Last but not least, if you ever want to do a lot of queries at the same time, you can take advantage of the Prisma transactions.