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and I don't know if this matters, but I did notice some difference in the generated SQL queries between the test code and the production one:
Test code log output
2018-07-23 15:25:20 INFO  LogHelper:31 - HHH000204: Processing PersistenceUnitInfo [
	name: templatePU
	...]
2018-07-23 15:25:20 INFO  Version:45 - HHH000412: Hibernate Core {5.2.17.Final}
2018-07-23 15:25:20 INFO  Environment:198 - HHH000205: Loaded properties from resource hibernate.properties: {hibernate.connection.driver_class=org.h2.Driver, javax.persistence.validation.mode=NONE, hibernate.service.allow_crawling=false, hibernate.max_fetch_depth=5, hibernate.dialect=org.hibernate.dialect.H2Dialect, hibernate.format_sql=true, hibernate.cache.region.factory_class=org.hibernate.testing.cache.CachingRegionFactory, hibernate.connection.username=sa, hibernate.cache.region_prefix=hibernate.test, hibernate.connection.url=jdbc:h2:mem:db1;DB_CLOSE_DELAY=-1, hibernate.bytecode.use_reflection_optimizer=false, hibernate.show_sql=false, hibernate.physical_naming_strategy=org.hibernate.bugs.hhh12641.ImprovedPhysicalNamingStrategyImpl, hibernate.session.events.log=true, hibernate.jdbc.batch_versioned_data=true, hibernate.connection.pool_size=5}
2018-07-23 15:25:20 WARN  CachingRegionFactory:52 - CachingRegionFactory should be only used for testing.
2018-07-23 15:25:20 INFO  Version:66 - HCANN000001: Hibernate Commons Annotations {5.0.1.Final}
2018-07-23 15:25:20 WARN  pooling:73 - HHH10001002: Using Hibernate built-in connection pool (not for production use!)
2018-07-23 15:25:20 INFO  pooling:129 - HHH10001005: using driver [org.postgresql.Driver] at URL [jdbc:postgresql://localhost:5432/c_application_rweb?binaryTransferDisable=TIMESTAMP]
2018-07-23 15:25:20 INFO  pooling:138 - HHH10001001: Connection properties: {user=c_application_rweb, password=****}
2018-07-23 15:25:20 INFO  pooling:143 - HHH10001003: Autocommit mode: false
2018-07-23 15:25:20 INFO  DriverManagerConnectionProviderImpl:281 - HHH000115: Hibernate connection pool size: 5 (min=1)
2018-07-23 15:25:20 INFO  Dialect:157 - HHH000400: Using dialect: org.hibernate.dialect.PostgreSQL95Dialect
2018-07-23 15:25:20 INFO  LobCreatorBuilderImpl:124 - HHH000424: Disabling contextual LOB creation as createClob() method threw error : java.lang.reflect.InvocationTargetException
2018-07-23 15:25:20 INFO  BasicTypeRegistry:148 - HHH000270: Type registration [java.util.UUID] overrides previous : org.hibernate.type.UUIDBinaryType@1162410a
2018-07-23 15:25:21 WARN  AnnotationBinder:872 - HHH000457: Joined inheritance hierarchy [org.hibernate.bugs.hhh12641.DataSourceResource] defined explicit @DiscriminatorColumn.  Legacy Hibernate behavior was to ignore the @DiscriminatorColumn.  However, as part of issue HHH-6911 we now apply the explicit @DiscriminatorColumn.  If you would prefer the legacy behavior, enable the `hibernate.discriminator.ignore_explicit_for_joined` setting (hibernate.discriminator.ignore_explicit_for_joined=true)
2018-07-23 15:25:21 INFO  access:47 - HHH10001501: Connection obtained from JdbcConnectionAccess [org.hibernate.engine.jdbc.env.internal.JdbcEnvironmentInitiator$ConnectionProviderJdbcConnectionAccess@3b4d50b] for (non-JTA) DDL execution was not in auto-commit mode; the Connection 'local transaction' will be committed and the Connection will be set into auto-commit mode.
2018-07-23 15:25:21 DEBUG SQL:92 - 
    insert 
    into
        data_source_resources
        (host, password, path, port, protocol, username, created_date, discriminator) 
    values
        (?, ?, ?, ?, ?, ?, ?, 'DATA_BASE')
Hibernate: 
    insert 
    into
        data_source_resources
        (host, password, path, port, protocol, username, created_date, discriminator) 
    values
        (?, ?, ?, ?, ?, ?, ?, 'DATA_BASE')
2018-07-23 15:25:21 TRACE BasicBinder:65 - binding parameter [1] as [VARCHAR] - [localhost]
2018-07-23 15:25:21 TRACE BasicBinder:53 - binding parameter [2] as [VARCHAR] - [null]
2018-07-23 15:25:21 TRACE BasicBinder:65 - binding parameter [3] as [VARCHAR] - [org/hibernate/bugs/hhh12641]
2018-07-23 15:25:21 TRACE BasicBinder:65 - binding parameter [4] as [INTEGER] - [0]
2018-07-23 15:25:21 TRACE BasicBinder:65 - binding parameter [5] as [VARCHAR] - [DataBaseSourceResource]
2018-07-23 15:25:21 TRACE BasicBinder:53 - binding parameter [6] as [VARCHAR] - [null]
2018-07-23 15:25:21 TRACE BasicBinder:65 - binding parameter [7] as [TIMESTAMP] - [Mon Jul 23 15:25:21 IDT 2018]
2018-07-23 15:25:22 DEBUG SQL:92 - 
    insert 
    into
        data_base_settings
        (db_name, db_type, maintenance_db, query, table_name, id) 
    values
        (?, ?, ?, ?, ?, ?)
Hibernate: 
    insert 
    into
        data_base_settings
        (db_name, db_type, maintenance_db, query, table_name, id) 
    values
        (?, ?, ?, ?, ?, ?)
2018-07-23 15:25:22 TRACE BasicBinder:65 - binding parameter [1] as [VARCHAR] - [DB-Mon Jul 23 15:25:21 IDT 2018]
2018-07-23 15:25:22 TRACE BasicBinder:65 - binding parameter [2] as [VARCHAR] - [MSSQL]
2018-07-23 15:25:22 TRACE BasicBinder:53 - binding parameter [3] as [VARCHAR] - [null]
2018-07-23 15:25:22 TRACE BasicBinder:65 - binding parameter [4] as [VARCHAR] - [SELECT COUNT(*) FROM DB-Mon Jul 23 15:25:21 IDT 2018]
2018-07-23 15:25:22 TRACE BasicBinder:53 - binding parameter [5] as [VARCHAR] - [null]
2018-07-23 15:25:22 TRACE BasicBinder:65 - binding parameter [6] as [BIGINT] - [4]
2018-07-23 15:25:22 INFO  StatisticalLoggingSessionEventListener:258 - Session Metrics {
    25588 nanoseconds spent acquiring 1 JDBC connections;
    22501 nanoseconds spent releasing 1 JDBC connections;
    471461 nanoseconds spent preparing 2 JDBC statements;
    2742434 nanoseconds spent executing 2 JDBC statements;
    0 nanoseconds spent executing 0 JDBC batches;
    0 nanoseconds spent performing 0 L2C puts;
    0 nanoseconds spent performing 0 L2C hits;
    0 nanoseconds spent performing 0 L2C misses;
    5898742 nanoseconds spent executing 1 flushes (flushing a total of 1 entities and 0 collections);
    0 nanoseconds spent executing 0 partial-flushes (flushing a total of 0 entities and 0 collections)
}
2018-07-23 15:25:22 INFO  pooling:233 - HHH10001008: Cleaning up connection pool [jdbc:postgresql://localhost:5432/c_application_rweb?binaryTransferDisable=TIMESTAMP]
Note that on the 2nd INSERT command the provided ID value is 4 (as it should be) but there is no previous
as shown in the production log (see one of the previous comments in the issue). So where did the 4 value come from ? How did the code know that the assigned value for the base entity was 4 and thus use it to insert the joined part ?