Add fine-grained migration APIs

[raymondfeng]

LB supports automigrate and autoupdate today. It will be useful to expose more fine-grained apis so that application developers can have better control of the process.

[faridnsh]

I'm also using postgres and I would love the following functionalities in autoupdate(Of course all optional):

• Old columns are not deleted, and if they have NOT NULL(required property/column), NOT NULL should be dropped(so that new data will work fine without it), mentioned in here: Prevent autoupdate dropping residual columns... strongloop/loopback#1942
• If column has a type change, the old data is casted to the new type whenever possible, if not possible then it should be either NULL or default value.
• A dry-run mode where the connector will only return a string which is the sql changes to be made without executing them, mentioned in here: Problem with autoupdate and automigrate strongloop/loopback#207
• If a new column is required and has a default value, then all the rows should have that default values(right now it fails), mentioned in here: When new required fields with a default value are added autoupdate shouldn't fail loopback-connector-postgresql#123.

It'd be nice if there would a flag for each of these functionalities.

[laurentdesmet]

+1 for this feature because now autoupdate can lead to dataloss because columns can be removed as mentioned above (issue strongloop/loopback#1942). Maybe the documentation should be updated because now this page states

When tables with data exist, use auto-update to avoid data loss.

which is a dangerous statement to make because autoupdate can actually remove data.

[e-part]

+1 - It would be great if I can see a SQL which shows only the changes made to the DB
before deciding whether it's safe to run it.

[stale]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.

[stale]

This issue has been closed due to continued inactivity. Thank you for your understanding. If you believe this to be in error, please contact one of the code owners, listed in the CODEOWNERS file at the top-level of this repository.