Support multiple columns in drop_constraint

[kvch]

Right now drop_constraint operation only supports dropping constraints that include a single column. With the introduction of create_constraint we now support creating constraints with multiple columns. Now we need to adjust drop_constraint.

[andrew-farries]

This will require a breaking change to the drop_constraint operation format.

Dropping a multi-column constraint will require down, and optionally up, data migrations to be specified for all columns covered by the constraint. This will mean the operation has to change from its current format:

[
    {
      "drop_constraint": {
        "table": "posts",
        "name": "title_length",
        "up": "title",
        "down": "(SELECT CASE WHEN length(title) <= 3 THEN LPAD(title, 4, '-') ELSE title END)"
      }
    }
]

to something that allows up and down data migrations for multiple columns:

[
    {
      "drop_constraint": {
        "table": "tickets",
        "name": "check_zip_name",
        "up": {
          "sellers_name": "sellers_name",
          "sellers_zip": "sellers_zip"
        },
        "down": {
          "sellers_name": "sellers_name",
          "sellers_zip": "sellers_zip"
        }
      }
    }
]

This new format looks similar to the create_constraint operation which also allows up and down migrations for multiple columns.

[andrew-farries]

Implemented in #487 as a new operation type drop_multicolumn_constraint rather than make the breaking change to drop_constraint.

#489 tracks the removal of the obsoletedrop_constraint operation.