Merge command hints to do a slice, but there is no option for this

[EnderASz]

Describe the bug

When I try to use alembic merge command to merge some non-head revisions together there appears an error with a hint to use a --slice option but when I try to use it makes clear that this option is not available for that command. It looks to me like it's just missing there, because as what I tested alembic handles mergepoints created from non-head revisions good enough when I write these revisions by my self.

alembic revision --rev-id a
alembic revision --rev-id b --head a
alembic revision --rev-id c --head b
alembic revision --rev-id d --head a --splice
alembic merge b d --rev-id e

results with

Generating /home/olek/Workspace/SPTech/RailyCargo/raily-cargo-db-migrations/src/versions/a_.py ...  done
Generating /home/olek/Workspace/SPTech/RailyCargo/raily-cargo-db-migrations/src/versions/b_.py ...  done
Generating /home/olek/Workspace/SPTech/RailyCargo/raily-cargo-db-migrations/src/versions/c_.py ...  done
Generating /home/olek/Workspace/SPTech/RailyCargo/raily-cargo-db-migrations/src/versions/d_.py ...  done
FAILED: Revision b is not a head revision; please specify --splice to create a new branch from this revision

and

alembic revision --rev-id a
alembic revision --rev-id b --head a
alembic revision --rev-id c --head b
alembic revision --rev-id d --head a --splice
alembic merge b d --rev-id e --splice

results with

Generating /home/olek/Workspace/SPTech/RailyCargo/raily-cargo-db-migrations/src/versions/a_.py ...  done Generating /home/olek/Workspace/SPTech/RailyCargo/raily-cargo-db-migrations/src/versions/b_.py ...  done
Generating /home/olek/Workspace/SPTech/RailyCargo/raily-cargo-db-migrations/src/versions/c_.py ...  done
Generating /home/olek/Workspace/SPTech/RailyCargo/raily-cargo-db-migrations/src/versions/d_.py ...  done
usage: alembic [-h] [--version] [-c CONFIG] [-n NAME] [-x X] [--raiseerr] [-q]
               {branches,check,current,downgrade,edit,ensure_version,heads,history,init,list_templates,merge,revision,show,stamp,upgrade} ...
alembic: error: unrecognized arguments: --splice

Versions.

• OS: Linux Pop!_OS 22.04 LTS
• Python: 3.12.11
• Alembic: 1.16.4
• SQLAlchemy: 2.0.29

[zzzeek]

hi -

--splice is not a command, the two dashes indicate this is a parameter to an existing command, in this case the revision command:

alembic revision -m "myrev" --head somehead --splice

[EnderASz]

--splice is not a command, the two dashes indicate this is a parameter to an existing command

Yeah - it was I meant to say. Sorry if i will not be clear enough sometimes

[zzzeek]

sorry is your bug report missing information? you have two sequences of command examples that look identical to me yet you claim different output, and the second output does not make any sense as it's indicating a command that was run.

converting to discussion since it's not clear what you are trying to do

[EnderASz]

Yeah - I just edited it right now. When I was writing this bug report I by mistake copied wrong file content saved on my PC.

I was meaning that command alembic merge in one case throws an error and hints that I can use --splice and when I try to use it it says that this is not possible.

[EnderASz]

does not make any sense as it's indicating a command that was run.

Yeah, it is kinda the point. When I try to run alembic merge with --splice regardles of other arguments it throws that --splice argument is not recognized.

[EnderASz]

It kinda looks like alembic merge and alembic revision commands use some common piece of code throwing this error - I'm not sure if this is a problem or lack of --splice possibility in alembic merge command is the problem.

[zzzeek]

if it were me looking at this, I'd say the only problem is that merge is outputting that particular error message which comes from the "create a revision" mechanics and it should be reworded in that codepath to say "this is not supported by merge". without looking into potential use cases here.

but then you are saying you wrote it all out as rev ids manually and "alembic is good enough"....which implies you're actually looking to make a merge point that adds a new head, rather than resolving two existing heads, and that....is beyond what I'd understand is actually useful in real world scenarios. github is having an outage so I cant search for if there was an issue to add --splice to merge. but at the same time it looks like --splice just disables a certain check from happening. But overall I dont know what happens if you tell it to make a new revision with multiple bases and those bases aren't heads, I guess it just does it? Here's the patch, it wont hurt on this end since it's nothing so feel free to try

diff --git a/alembic/command.py b/alembic/command.py
index 8e48547..ff9b159 100644
--- a/alembic/command.py
+++ b/alembic/command.py
@@ -385,6 +385,7 @@ def merge(
     message: Optional[str] = None,
     branch_label: Optional[_RevIdType] = None,
     rev_id: Optional[str] = None,
+    splice: bool = False,
 ) -> Optional[Script]:
     """Merge two revisions together.  Creates a new migration file.
 
@@ -435,6 +436,7 @@ def merge(
         refresh=True,
         head=revisions,
         branch_labels=branch_label,
+        splice=splice,
         **template_args,  # type:ignore[arg-type]
     )

[CaselIT]

@zzzeek do you want to implement this change? if so I can open an issue to track it

[zzzeek]

Yes we should do it. Super small change

[CaselIT]

ok, tracked by #1712