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Task: Implement a save method of a simple ORM
* create findById * create ORM annotations * setup a simple infrastructure (init DataSource) * prep save() method as a key todo
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‎lesson-demo/pom.xml

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<modelVersion>4.0.0</modelVersion>
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<artifactId>lesson-demo</artifactId>
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<dependencies>
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<dependency>
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<groupId>org.postgresql</groupId>
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<artifactId>postgresql</artifactId>
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<version>42.7.3</version>
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</dependency>
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<dependency>
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<groupId>com.google.guava</groupId>
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<artifactId>guava</artifactId>
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<version>33.2.0-jre</version>
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</dependency>
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</dependencies>
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</project>

‎lesson-demo/src/main/java/com/bobocode/DemoApp.java

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package com.bobocode;
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import com.bobocode.bibernate.OrmImpl;
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import com.bobocode.entity.Participant;
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public class DemoApp {
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public static void main(String[] args) {
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var orm = new OrmImpl("jdbc:postgresql://0.tcp.eu.ngrok.io:11026/postgres", "bobouser", "bobopass");
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var participant = orm.findById(Participant.class, 23);
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System.out.println(participant);
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}
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}

‎lesson-demo/src/main/java/com/bobocode/bibernate/Orm.java

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package com.bobocode.bibernate;
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public interface Orm {
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<T> T findById(Class<T> entityType, Object id);
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void save(Object entity);
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}

‎lesson-demo/src/main/java/com/bobocode/bibernate/OrmImpl.java

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package com.bobocode.bibernate;
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import com.bobocode.bibernate.annotation.Column;
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import com.bobocode.bibernate.annotation.Entity;
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import com.bobocode.bibernate.annotation.Id;
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import com.bobocode.bibernate.annotation.Table;
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import com.google.common.base.CaseFormat;
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import lombok.RequiredArgsConstructor;
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import lombok.SneakyThrows;
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import org.postgresql.ds.PGSimpleDataSource;
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import javax.sql.DataSource;
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import java.lang.reflect.Field;
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import java.sql.ResultSet;
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import java.sql.Timestamp;
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import java.util.Arrays;
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import java.util.Optional;
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@RequiredArgsConstructor
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public class OrmImpl implements Orm {
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public static final String SELECT_FROM_TABLE_BY_COLUMN = "select * from %s where %s = ?";
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private final DataSource dataSource;
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public OrmImpl(String jdbcUrl, String username, String password) {
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var pgSimpleDataSource = new PGSimpleDataSource();
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pgSimpleDataSource.setURL(jdbcUrl);
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pgSimpleDataSource.setUser(username);
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pgSimpleDataSource.setPassword(password);
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this.dataSource = pgSimpleDataSource;
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}
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@Override
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@SneakyThrows
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public <T> T findById(Class<T> entityType, Object id) {
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verifyEntity(entityType);
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try (var connection = dataSource.getConnection()) {
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var selectSql = buildSqlSelectById(entityType);
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System.out.println("SQL: " + selectSql);
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try (var selectStatement = connection.prepareStatement(selectSql)) {
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selectStatement.setObject(1, id);
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var rs = selectStatement.executeQuery();
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if (rs.next()) {
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return createEntityFromResultSet(entityType, rs);
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} else {
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throw new RuntimeException("Entity not found by id =" + id);
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}
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}
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}
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}
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private void verifyEntity(Class<?> entityType) {
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if (!entityType.isAnnotationPresent(Entity.class)) {
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throw new RuntimeException(entityType.getSimpleName() + " is not an @Entity");
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}
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}
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private String buildSqlSelectById(Class<?> entityType) {
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var tableName = resolveTableName(entityType);
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var idColumnName = resolveIdColumnName(entityType);
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return SELECT_FROM_TABLE_BY_COLUMN.formatted(tableName, idColumnName);
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}
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private String resolveTableName(Class<?> entityType) {
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return Optional.ofNullable(entityType.getAnnotation(Table.class))
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.map(Table::value)
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.orElseGet(() -> underscore(entityType.getSimpleName()));
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}
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private String resolveIdColumnName(Class<?> entityType) {
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return Arrays.stream(entityType.getDeclaredFields())
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.filter(field -> field.isAnnotationPresent(Id.class))
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.findAny()
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.map(this::resolveColumnName)
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.orElseThrow(() -> new RuntimeException("Entity " + entityType.getSimpleName() + " must have an @Id"));
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}
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private String resolveColumnName(Field field) {
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return Optional.ofNullable(field.getAnnotation(Column.class))
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.map(Column::value)
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.orElseGet(() -> underscore(field.getName()));
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}
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private String underscore(String value) {
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return CaseFormat.LOWER_CAMEL
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.converterTo(CaseFormat.LOWER_UNDERSCORE)
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.convert(value);
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}
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@SneakyThrows
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private <T> T createEntityFromResultSet(Class<T> entityType, ResultSet rs) {
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var entity = entityType.getConstructor().newInstance();
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for (var field : entityType.getDeclaredFields()) {
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var columnName = resolveColumnName(field);
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var fieldValue = rs.getObject(columnName);
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if (fieldValue instanceof Timestamp timestamp) {
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fieldValue = timestamp.toLocalDateTime();
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}
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field.setAccessible(true);
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field.set(entity, fieldValue);
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}
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return entity;
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}
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/**
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* Saves the given entity by persisting it in the database and sets the id value which is generated by the DB.
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*
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* @param entity the entity to be saved
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* @throws RuntimeException if the method is not implemented
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*/
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@Override
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public void save(Object entity) {
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throw new UnsupportedOperationException("Method save() is not implemented yet"); // todo: implement this method and remove the exception
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}
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}

‎lesson-demo/src/main/java/com/bobocode/bibernate/annotation/Column.java

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package com.bobocode.bibernate.annotation;
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import java.lang.annotation.ElementType;
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import java.lang.annotation.Retention;
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import java.lang.annotation.RetentionPolicy;
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import java.lang.annotation.Target;
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@Retention(RetentionPolicy.RUNTIME)
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@Target(ElementType.FIELD)
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public @interface Column {
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String value();
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boolean updatable() default true;
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}

‎lesson-demo/src/main/java/com/bobocode/bibernate/annotation/Entity.java

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package com.bobocode.bibernate.annotation;
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import java.lang.annotation.ElementType;
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import java.lang.annotation.Retention;
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import java.lang.annotation.RetentionPolicy;
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import java.lang.annotation.Target;
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@Retention(RetentionPolicy.RUNTIME)
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@Target(ElementType.TYPE)
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public @interface Entity {
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}

‎lesson-demo/src/main/java/com/bobocode/bibernate/annotation/Id.java

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package com.bobocode.bibernate.annotation;
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import java.lang.annotation.ElementType;
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import java.lang.annotation.Retention;
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import java.lang.annotation.RetentionPolicy;
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import java.lang.annotation.Target;
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@Retention(RetentionPolicy.RUNTIME)
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@Target(ElementType.FIELD)
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public @interface Id {
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}

‎lesson-demo/src/main/java/com/bobocode/bibernate/annotation/Table.java

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package com.bobocode.bibernate.annotation;
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import java.lang.annotation.ElementType;
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import java.lang.annotation.Retention;
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import java.lang.annotation.RetentionPolicy;
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import java.lang.annotation.Target;
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@Retention(RetentionPolicy.RUNTIME)
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@Target(ElementType.TYPE)
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public @interface Table {
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String value();
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}

‎lesson-demo/src/main/java/com/bobocode/entity/Participant.java

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package com.bobocode.entity;
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import com.bobocode.bibernate.annotation.Column;
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import com.bobocode.bibernate.annotation.Entity;
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import com.bobocode.bibernate.annotation.Id;
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import com.bobocode.bibernate.annotation.Table;
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import lombok.Data;
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import lombok.NoArgsConstructor;
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import java.time.LocalDateTime;
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@Entity
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@Table("participants")
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@Data
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@NoArgsConstructor
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public class Participant {
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@Id
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private Integer id;
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private String firstName;
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private String lastName;
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private String city;
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private String company;
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private String position;
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private Integer yearsOfExperience;
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@Column(value = "created_at")
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private LocalDateTime createdAt;
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}

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