java - What properties does @Column columnDefinition make redundant?

Question

I often specify my @Column annotations like this:

@Column(columnDefinition="character varying (100) not null",length=100,nullable=false)

As you can see I specify length and nullable even though the columnDefinition already specifies those. That's because I don't know where/when these values are used exactly.

So, when specifying columnDefinition, what other properties of @Column are made redundant?

If it matters, I use Hibernate and PostgreSQL

Answer 1

My Answer: All of the following should be overridden (i.e. describe them all within columndefinition, if appropriate):

• length
• precision
• scale
• nullable
• unique

i.e. the column DDL will consist of: name + columndefinition and nothing else.

Rationale follows.

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1. Annotation containing the word "Column" or "Table" is purely physical - properties only used to control DDL/DML against database.

2. Other annotation purely logical - properties used in-memory in java to control JPA processing.

3. That's why sometimes it appears the optionality/nullability is set twice - once via @Basic(...,optional=true) and once via @Column(...,nullable=true). Former says attribute/association can be null in the JPA object model (in-memory), at flush time; latter says DB column can be null. Usually you'd want them set the same - but not always, depending on how the DB tables are setup and reused.

In your example, length and nullable properties are overridden and redundant.

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So, when specifying columnDefinition, what other properties of @Column are made redundant?

1. In JPA Spec & javadoc:

   • columnDefinition definition: The SQL fragment that is used when generating the DDL for the column.

   • columnDefinition default: Generated SQL to create a column of the inferred type.

   • The following examples are provided:

     @Column(name="DESC", columnDefinition="CLOB NOT NULL", table="EMP_DETAIL")
     @Column(name="EMP_PIC", columnDefinition="BLOB NOT NULL")
     

   • And, err..., that's it really. :-$ ?!

   Does columnDefinition override other properties provided in the same annotation?

   The javadoc and JPA spec don't explicity address this - spec's not giving great protection. To be 100% sure, test with your chosen implementation.

2. The following can be safely implied from examples provided in the JPA spec

   • name & table can be used in conjunction with columnDefinition, neither are overridden
   • nullable is overridden/made redundant by columnDefinition

3. The following can be fairly safely implied from the "logic of the situation" (did I just say that?? :-P ):

   • length, precision, scale are overridden/made redundant by the columnDefinition - they are integral to the type
   • insertable and updateable are provided separately and never included in columnDefinition, because they control SQL generation in-memory, before it is emmitted to the database.

4. That leaves just the "unique" property. It's similar to nullable - extends/qualifies the type definition, so should be treated integral to type definition. i.e. should be overridden.

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Test My Answer For columns "A" & "B", respectively:

  @Column(name="...", table="...", insertable=true, updateable=false,
          columndefinition="NUMBER(5,2) NOT NULL UNIQUE"

  @Column(name="...", table="...", insertable=false, updateable=true,
          columndefinition="NVARCHAR2(100) NULL"

• confirm generated table has correct type/nullability/uniqueness
• optionally, do JPA insert & update: former should include column A, latter column B