[BAEL-4056] Guide to MultipleBagFetchException (#10294)

* Adds Domain Models
* Adds Integration Tests

persistence-modules/java-jpa-3/src/main/java/com/baeldung/jpa/multiplebagfetchexception/Album.java

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+package com.baeldung.jpa.multiplebagfetchexception;
+
+import javax.persistence.Entity;
+import javax.persistence.GeneratedValue;
+import javax.persistence.GenerationType;
+import javax.persistence.Id;
+import javax.persistence.ManyToMany;
+import javax.persistence.OneToMany;
+import java.util.List;
+import java.util.Objects;
+import java.util.Set;
+
+@Entity
+class Album {
+
+    @Id
+    @GeneratedValue(strategy = GenerationType.AUTO)
+    private Long id;
+
+    private String name;
+
+    @OneToMany(mappedBy = "album")
+    private List<Song> songs;
+
+    @ManyToMany(mappedBy = "followingAlbums")
+    private Set<User> followers;
+
+    Album(String name) {
+        this.name = name;
+    }
+
+    @Override
+    public boolean equals(Object o) {
+        if (this == o)
+            return true;
+        if (o == null || getClass() != o.getClass())
+            return false;
+
+        Album album = (Album) o;
+
+        return Objects.equals(id, album.id);
+    }
+
+    @Override
+    public int hashCode() {
+        return id != null ? id.hashCode() : 0;
+    }
+
+    protected Album() {
+    }
+
+}

persistence-modules/java-jpa-3/src/main/java/com/baeldung/jpa/multiplebagfetchexception/Artist.java

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+package com.baeldung.jpa.multiplebagfetchexception;
+
+import javax.persistence.CascadeType;
+import javax.persistence.Entity;
+import javax.persistence.GeneratedValue;
+import javax.persistence.GenerationType;
+import javax.persistence.Id;
+import javax.persistence.OneToMany;
+import java.util.ArrayList;
+import java.util.List;
+import java.util.Objects;
+
+@Entity
+class Artist {
+
+    @Id
+    @GeneratedValue(strategy = GenerationType.AUTO)
+    private Long id;
+
+    private String name;
+
+    @OneToMany(mappedBy = "artist")
+    private List<Song> songs;
+
+    @OneToMany(mappedBy = "artist", cascade = CascadeType.PERSIST)
+    private List<Offer> offers;
+
+    Artist(String name) {
+        this.name = name;
+        this.offers = new ArrayList<>();
+    }
+
+    void createOffer(String description) {
+        this.offers.add(new Offer(description, this));
+    }
+
+    @Override
+    public boolean equals(Object o) {
+        if (this == o)
+            return true;
+        if (o == null || getClass() != o.getClass())
+            return false;
+
+        Artist artist = (Artist) o;
+
+        return Objects.equals(id, artist.id);
+    }
+
+    @Override
+    public int hashCode() {
+        return id != null ? id.hashCode() : 0;
+    }
+
+    protected Artist() {
+    }
+}

persistence-modules/java-jpa-3/src/main/java/com/baeldung/jpa/multiplebagfetchexception/DummyEntity.java

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+package com.baeldung.jpa.multiplebagfetchexception;
+
+import javax.persistence.ElementCollection;
+import javax.persistence.Entity;
+import javax.persistence.GeneratedValue;
+import javax.persistence.GenerationType;
+import javax.persistence.Id;
+import java.util.List;
+import java.util.Objects;
+
+@Entity
+class DummyEntity {
+
+    @Id
+    @GeneratedValue(strategy = GenerationType.AUTO)
+    private Long id;
+
+    @ElementCollection
+    private List<String> collection1;
+
+    @ElementCollection
+    private List<String> collection2;
+
+    @Override
+    public boolean equals(Object o) {
+        if (this == o)
+            return true;
+        if (o == null || getClass() != o.getClass())
+            return false;
+
+        DummyEntity that = (DummyEntity) o;
+
+        return Objects.equals(id, that.id);
+    }
+
+    @Override
+    public int hashCode() {
+        return id != null ? id.hashCode() : 0;
+    }
+
+    protected DummyEntity() {
+    }
+}

persistence-modules/java-jpa-3/src/main/java/com/baeldung/jpa/multiplebagfetchexception/FavoriteSong.java

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+package com.baeldung.jpa.multiplebagfetchexception;
+
+import javax.persistence.Column;
+import javax.persistence.Entity;
+import javax.persistence.GeneratedValue;
+import javax.persistence.GenerationType;
+import javax.persistence.Id;
+import javax.persistence.ManyToOne;
+import java.util.Objects;
+
+@Entity
+class FavoriteSong {
+
+    @Id
+    @GeneratedValue(strategy = GenerationType.AUTO)
+    private Long id;
+
+    @ManyToOne
+    private Song song;
+
+    @ManyToOne
+    private User user;
+
+    @Column(name = "arrangement_index", nullable = false)
+    private int arrangementIndex;
+
+    FavoriteSong(Song song, User user) {
+        this.song = song;
+        this.user = user;
+    }
+
+    @Override
+    public boolean equals(Object o) {
+        if (this == o)
+            return true;
+        if (o == null || getClass() != o.getClass())
+            return false;
+
+        FavoriteSong likedSong = (FavoriteSong) o;
+
+        return Objects.equals(id, likedSong.id);
+    }
+
+    @Override
+    public int hashCode() {
+        return id != null ? id.hashCode() : 0;
+    }
+
+    protected FavoriteSong() {
+    }
+
+}

persistence-modules/java-jpa-3/src/main/java/com/baeldung/jpa/multiplebagfetchexception/Offer.java

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+package com.baeldung.jpa.multiplebagfetchexception;
+
+import javax.persistence.Entity;
+import javax.persistence.GeneratedValue;
+import javax.persistence.GenerationType;
+import javax.persistence.Id;
+import javax.persistence.ManyToOne;
+
+@Entity
+class Offer {
+
+    @Id
+    @GeneratedValue(strategy = GenerationType.AUTO)
+    private Long id;
+
+    private String name;
+
+    @ManyToOne
+    private Artist artist;
+
+    Offer(String name, Artist artist) {
+        this.name = name;
+        this.artist = artist;
+    }
+
+    @Override
+    public boolean equals(Object o) {
+        if (this == o)
+            return true;
+        if (o == null || getClass() != o.getClass())
+            return false;
+
+        Offer offer = (Offer) o;
+
+        return id != null ? id.equals(offer.id) : offer.id == null;
+    }
+
+    @Override
+    public int hashCode() {
+        return id != null ? id.hashCode() : 0;
+    }
+
+    protected Offer() {
+    }
+
+}

persistence-modules/java-jpa-3/src/main/java/com/baeldung/jpa/multiplebagfetchexception/Playlist.java

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+package com.baeldung.jpa.multiplebagfetchexception;
+
+import javax.persistence.Entity;
+import javax.persistence.GeneratedValue;
+import javax.persistence.GenerationType;
+import javax.persistence.Id;
+import javax.persistence.ManyToOne;
+import java.util.Objects;
+
+@Entity
+class Playlist {
+
+    @Id
+    @GeneratedValue(strategy = GenerationType.AUTO)
+    private Long id;
+
+    private String name;
+
+    @ManyToOne
+    private User createdBy;
+
+    Playlist(String name, User createdBy) {
+        this.name = name;
+        this.createdBy = createdBy;
+    }
+
+    @Override
+    public boolean equals(Object o) {
+        if (this == o)
+            return true;
+        if (o == null || getClass() != o.getClass())
+            return false;
+
+        Playlist playlist = (Playlist) o;
+
+        return Objects.equals(id, playlist.id);
+    }
+
+    @Override
+    public int hashCode() {
+        return id != null ? id.hashCode() : 0;
+    }
+
+    protected Playlist() {
+    }
+
+}

persistence-modules/java-jpa-3/src/main/java/com/baeldung/jpa/multiplebagfetchexception/Song.java

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+package com.baeldung.jpa.multiplebagfetchexception;
+
+import javax.persistence.Entity;
+import javax.persistence.GeneratedValue;
+import javax.persistence.GenerationType;
+import javax.persistence.Id;
+import javax.persistence.ManyToOne;
+import java.util.Objects;
+
+@Entity
+class Song {
+
+    @Id
+    @GeneratedValue(strategy = GenerationType.AUTO)
+    private Long id;
+
+    private String name;
+
+    @ManyToOne
+    private Album album;
+
+    @ManyToOne
+    private Artist artist;
+
+    Song(String name, Artist artist) {
+        this.name = name;
+        this.artist = artist;
+    }
+
+    void assignToAlbum(Album album) {
+        this.album = album;
+    }
+
+    @Override
+    public boolean equals(Object o) {
+        if (this == o)
+            return true;
+        if (o == null || getClass() != o.getClass())
+            return false;
+
+        Song song = (Song) o;
+
+        return Objects.equals(id, song.id);
+    }
+
+    @Override
+    public int hashCode() {
+        return id != null ? id.hashCode() : 0;
+    }
+
+    protected Song() {
+    }
+
+}
+ 

persistence-modules/java-jpa-3/src/main/java/com/baeldung/jpa/multiplebagfetchexception/User.java

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+package com.baeldung.jpa.multiplebagfetchexception;
+
+import javax.persistence.CascadeType;
+import javax.persistence.Entity;
+import javax.persistence.GeneratedValue;
+import javax.persistence.GenerationType;
+import javax.persistence.Id;
+import javax.persistence.ManyToMany;
+import javax.persistence.OneToMany;
+import javax.persistence.OrderColumn;
+import java.util.ArrayList;
+import java.util.HashSet;
+import java.util.List;
+import java.util.Objects;
+import java.util.Set;
+
+@Entity
+class User {
+
+    @Id
+    @GeneratedValue(strategy = GenerationType.AUTO)
+    private Long id;
+
+    private String name;
+
+    @OneToMany(mappedBy = "createdBy", cascade = CascadeType.PERSIST)
+    private List<Playlist> playlists;
+
+    @OneToMany(mappedBy = "user", cascade = CascadeType.PERSIST)
+    @OrderColumn(name = "arrangement_index")
+    private List<FavoriteSong> favoriteSongs;
+
+    @ManyToMany
+    private Set<Album> followingAlbums;
+
+    User(String name) {
+        this.name = name;
+        this.playlists = new ArrayList<>();
+        this.favoriteSongs = new ArrayList<>();
+        this.followingAlbums = new HashSet<>();
+    }
+
+    void followAlbum(Album album) {
+        this.followingAlbums.add(album);
+    }
+
+    void createPlaylist(String name) {
+        this.playlists.add(new Playlist(name, this));
+    }
+
+    void addSongToFavorites(Song song) {
+        this.favoriteSongs.add(new FavoriteSong(song, this));
+    }
+
+    @Override
+    public boolean equals(Object o) {
+        if (this == o)
+            return true;
+        if (o == null || getClass() != o.getClass())
+            return false;
+
+        User user = (User) o;
+
+        return Objects.equals(id, user.id);
+    }
+
+    @Override
+    public int hashCode() {
+        return id != null ? id.hashCode() : 0;
+    }
+
+    protected User() {
+    }
+}