@Entity
public class Episode {

    public Episode() {}

    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Long id;
    ....
    @ManyToMany(cascade = {CascadeType.PERSIST, CascadeType.MERGE})
    @JoinTable(name = "episode_person", joinColumns = @JoinColumn(name = "episode_id", referencedColumnName = "id"), inverseJoinColumns = @JoinColumn(name = "person_id", referencedColumnName = "id"))
    private List<EpisodePerson> persons;
    @OneToOne(cascade=CascadeType.ALL)
    private Address address;

    // getters setters

        public List<EpisodePerson> getPersons() {
        return persons;
    }

@Entity
public class Person {

    public Person() {
    }

    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Long id;
    ....
    @JsonIgnore
    @ManyToMany(mappedBy = "persons", cascade = {CascadeType.PERSIST, CascadeType.MERGE})
    private List<EpisodePerson> episodes;

    // getters and setters

    public List<EpisodePerson> getEpisodes() {
        return episodes;
    }

@Entity
public class EpisodePerson {

    @EmbeddedId
    private EpisodePersonId id;

    @ManyToOne
    @JoinColumn(name = "fk_episode", insertable = false, updatable = false)
    private Episode episode;

    @ManyToOne
    @JoinColumn(name = "fk_person", insertable = false, updatable = false)
    private Person person;

    public enum EpisodeRole {
        Goodie,
        Baddie,
        Helper
    }

    @Column
    @Enumerated(EnumType.STRING)
    private EpisodeRole episodeRole;

    public EpisodePerson(Episode e, Person p, EpisodeRole r) {
        // create primary key
        this.id = new EpisodePersonId(e.getId(), p.getId());

        // initialize attributes
        this.episode = e;
        this.person = p;
        this.episodeRole = r;

        // update relationships to assure referential integrity
        p.getEpisodes().add(this);
        e.getPersons().add(this);
    }
}

    @Override
    @Transactional
    public Episode saveEpisode(Episode episode) {

        // CHECK FOR EXISTING PEOPLE ON NID
        List mergedPeople = personService.mergeDetachedWithAttached( episode.getPersons() ); << --- not the input parameter required type
        episode.setPersons( mergedPeople );

    ...
[code]

mergeDetachedWithAttached uses episode.getPersons() - but the getPersons method returns

[code]
        public List<EpisodePerson> getPersons() {
        return persons;
    }

@Entity
public class Person {

    public Person() {
    }

    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Long id;
    // other stuff
    @JsonIgnore
    @OneToMany(cascade = {CascadeType.PERSIST, CascadeType.MERGE})
    private List<EpisodePerson> episodes;

    // getters setters

    public void setEpisodes(List<EpisodePerson> episodes) {
        this.episodes = episodes;
    }

    public List<EpisodePerson> getEpisodes() {
        return episodes;
    }
}

package com.example.dao;

//import org.hibernate.search.annotations.Field;
//import org.hibernate.search.annotations.Indexed;
import org.hibernate.validator.constraints.NotEmpty;
import org.springframework.format.annotation.DateTimeFormat;

import javax.persistence.*;
import javax.validation.constraints.Pattern;
import java.util.ArrayList;
import java.util.Date;
import java.util.List;

@Entity
//@Indexed
public class Episode {

    public Episode() {}

    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Long id;
     
    // other stuff

    @OneToMany(cascade = {CascadeType.PERSIST, CascadeType.MERGE})
    private List<EpisodePerson> persons;
   
    // getters setters

    public List<EpisodePerson> getPersons() {
        return persons;
    }

    public void setPersons(List<EpisodePerson> persons) {
        this.persons = persons;
    }
}

@Entity
public class EpisodePerson {

    @EmbeddedId
    private EpisodePersonId id;

    @ManyToOne
    @JoinColumn(name = "fk_episode", insertable = false, updatable = false)
    private Episode episode;

    @ManyToOne
    @JoinColumn(name = "fk_person", insertable = false, updatable = false)
    private Person person;

    public enum EpisodeRole {
         Goodie,
         Baddie
    }

    @Column
    @Enumerated(EnumType.STRING)
    private EpisodeRole episodeRole;

    @EmbeddedId
    public EpisodePersonId getPk() {                 <<<<<<----
        return id;
    }

    @Transient
    public Episode getEpisode() {                   <<<<<<<------
        return getPk().getEpisode();
    }

    public EpisodePerson(Episode e, Person p, EpisodeRole r) {
        // create primary key
        this.id = new EpisodePersonId(e.getId(), p.getId());

        // initialize attributes
        this.episode = e;
        this.person = p;
        this.episodeRole = r;

        // update relationships to assure referential integrity
        p.getEpisodes().add(this);
        e.getPersons().add(this);
    }
}

    @Embeddable
    public class EpisodePersonId implements Serializable {

        @Column(name = "fk_episode")
        protected Long episodeId;

        @Column(name = "fk_person")
        protected Long personId;

        public EpisodePersonId() { }

        private Episode episode;           <<<<<<<---
        private Person person;              <<<<<<<---

        @ManyToOne
        public Episode getEpisode() {      <<<<<<----
            return episode;
        }

        public EpisodePersonId(Long episodeId, Long personId) {
            this.episodeId = episodeId;
            this.personId = personId;
        }

        @Override
        public int hashCode() {
            final int prime = 31;
            int result = 1;
            result = prime * result
                    + ((episodeId == null) ? 0 : episodeId.hashCode());
            result = prime * result
                    + ((personId == null) ? 0 : personId.hashCode());
            return result;
        }

        @Override
        public boolean equals(Object obj) {
            if (this == obj)
                return true;
            if (obj == null)
                return false;
            if (getClass() != obj.getClass())
                return false;

            EpisodePersonId other = (EpisodePersonId) obj;

            if (episodeId == null) {
                if (other.episodeId != null)
                    return false;
            } else if (!episodeId.equals(other.episodeId))
                return false;

            if (personId == null) {
                if (other.personId != null)
                    return false;
            } else if (!personId.equals(other.personId))
                return false;
            return true;
        }
    }