"org.springframework.beans.ConversionNotSupportedException" Error in spring while inserting data into Database

Question

I am saving the data to the table in database but it is showing me error like

Resolved [org.springframework.beans.ConversionNotSupportedException: Failed to convert property value of type 'java.lang.Long' to required type 'com.ashwin.springsecurityangular.model.Letter' for property 'letter'; nested exception is java.lang.IllegalStateException: Cannot convert value of type 'java.lang.Long' to required type 'com.ashwin.springsecurityangular.model.Letter' for property 'letter': no matching editors or conversion strategy found]

Letter.java class

 @Entity
    @Table(name = "letter")
    public class Letter implements Serializable {


        /**
         * 
         */
        private static final long serialVersionUID = 1L;


        @Id
        @GeneratedValue(strategy = GenerationType.AUTO)
        private long letterNo;

        @ManyToOne(fetch = FetchType.EAGER, optional = false,cascade=CascadeType.PERSIST)
        @JoinColumn(name = "letterId")
        @JsonIgnore
        private ClkLetter clkletter;

        private String inOut;

        private String inOutNo;

        private String inOutDate;

        private String letterIssuedSubBy;

        private String letterFile;

        private String representativeName;

        @ManyToOne(fetch = FetchType.EAGER, optional = false,cascade=CascadeType.PERSIST)
        @JoinColumn(name = "selectionId")
        @JsonIgnore
        private Selection selection;

        @ManyToOne(fetch = FetchType.EAGER, optional = false,cascade=CascadeType.PERSIST)
        @JoinColumn(name = "assessmentNo")
        @JsonIgnore
        private Assessment assessment;


        public Letter() {

        }
//i omitted all getters and setters
    }

LetterDoc.java

Here in this table i haven't used a single column which is like @Id type we used in making normal table.Here I have used two primary key as they belong to the foreign key of Letter and Document Class.

@Entity
@Table(name = "letter_doc")
@IdClass(AssignedRoleId.class)
public class LetterDoc implements Serializable {


    private static final long serialVersionUID = 1L;

    @Id
    @ManyToOne(fetch = FetchType.EAGER, optional = false,cascade=CascadeType.PERSIST)
    @JoinColumn(name = "letterNo",unique=true)
    @JsonIgnore
    private Letter letter;

    @Id
    @ManyToOne(fetch = FetchType.EAGER, optional = false,cascade=CascadeType.PERSIST)
    @JoinColumn(name = "documentId",unique=true)
    @JsonIgnore
    private Document document;

    private String docFile;

    public LetterDoc(Letter letter, Document document, String docFile) {

        this.letter = letter;
        this.document = document;
        this.docFile = docFile;
    }

    public Letter getLetter() {
        return letter;
    }

    public void setLetter(Letter letter) {
        this.letter = letter;
    }

    public Document getDocument() {
        return document;
    }

    public void setDocument(Document document) {
        this.document = document;
    }

    public String getDocFile() {
        return docFile;
    }

    public void setDocFile(String docFile) {
        this.docFile = docFile;
    }

    public LetterDoc() {

    }



     }

AssignedRoleId.java

public class AssignedRoleId implements Serializable {

    private Letter letter;
    private Document document;

    public AssignedRoleId(Letter letter, Document document) {

        this.letter = letter;
        this.document = document;
    }

    public AssignedRoleId() {}

    @Override
    public boolean equals(Object o) {
         if (o == this) {
             return true;
         }
         if (!(o instanceof LetterDoc)) {
             return false;
         }
         LetterDoc assignedRole = (LetterDoc) o;
         return Objects.equals(letter, assignedRole.getLetter()) &&
                 Objects.equals(document, assignedRole.getDocument());

    }

    @Override
    public int hashCode() {
        return Objects.hash(letter, document);
    }

    public Letter getLetter() {
        return letter;
    }

    public void setLetter(Letter letter) {
        this.letter = letter;
    }

    public Document getDocument() {
        return document;
    }

    public void setDocument(Document document) {
        this.document = document;
    }

I am trying to insert the data like using:

At first the data in Letter table gets inserted.Here is no problem as it is successfully inserted:

Letter letter=letterRepository.save(new Letter(clkLetter,sldto.getLetterDto().getInOut(),sldto.getLetterDto().getInOutNo(),sldto.getLetterDto().getInOutDate(),sldto.getLetterDto().getLetterIssuedSubBy(),sldto.getLetterDto().getLetterFile(),sldto.getLetterDto().getRepresentativeName()
                ,selection,assessment));

The i tried to insert the data in letter_doc table but it is not happening. document table is simply a look up table.

Document dox=documentRepository.findById((long)documentDto.getDocId()).get();
                 letterDocRepository.save(new LetterDoc(letter,dox,"a"));

My tables in database looks like this: