ENH: Support different ColumnDefinition for different database platforms

[nPraml]

Hi @rbygrave ,

with db2 we have the problem, we define columns as blob (without length):

Java Code:

  public class ReportResult(
  ...
  @Lob
  private File fileContent;
  ...
  )

generated init migration:

  create table report_result(
  ...
  file_content      blob,
  ...
  )

In this case we can only save a file that is not bigger as 1 MB (default size of db2 blob).
We would like to define the columnDefinition for a field more precise: different length for different platforms.

We would like to create a new annotation (e.g. DbColumn?) which uses the javax.persistence.@Column as a base and can be defined for every platform different:

  public class ReportResult(
  ...
  @Lob
  @DbColumn(columnDefinition = "blob(64M)", platform = Platfrom.DB2)
  // for all other platforms default generated type
  private File fileContent;
  ...
  )

What do you think about it?

[nPraml]

I tried to implement the new annotation, but it would be a huge modification. The xmls of the column definitions had to be also modified (list of platform-columnDefinition pairs) and lot of code in ebean.

We designed a new proposed solution which has only small code changes in ebean but it "redefines" the usage of @Column. (It's backward compatible)

Example:

 @Column(columnDefinition = "db2;blob(64M);sqlserver,h2;varchar(227);varchar(127)")
  String description;

Using the pattern

 @Column(columnDefinition = "PLATFORM1;DEFINITION1;PLATFORM2,PLATFORM3;DEFINITON2;DEFINITON-DEFAULT")

We use semicolon as separator because it's very unlikely that it would be used for column definition.

You can check the ebean code change here: FOCONIS@3d83c6b

What do you think about this?

[rbygrave]

I like it - nice approach !!

[nPraml]

Thank you! I opened a PR for it: #2563

[rbygrave]

Closing with the merged PR. Nice work !!