Practical advice: If you want to/vote to migrate something to another site (especially math.SE) please make sure that at least one of the tags on the question exists on the target site (math.SE). Possibly, do a retag to achieve this. If not, the migration will be rejected.

Update: The request on meta.SO got declined. I think there is not very much for MO to be done, as mainly we migrate to math.SE and the problem can reasonably be only fixed there. (To fix the issue on the target site, is also the 'official solution', see Shog9's reply on meta.SO.) Fortunately, there is already some activity on this on meta.math.SE.

Original post:

I just learned that a migration is automatically rejected if none of its tags exists on the target site.

This seems like a problem to me, since while math.SE evidently has tags for group theory, number theory and so on, they are not identically named as here on MO (since we have the two letter prefixes, mainly).

And, apparently, tag-synonyms are not even taken into account (as a question then tagged which has as synonym got autorejected, while that later tag exists on math.SE).

Personally, I think this automatic "sanity check" based on tags is not a good idea, and would simply drop it. This however might be complicated to push-through, so a more modest suggestion:

Include tag-synonyms in this automatic check.

In this way, it would be possible to make the most common tags compatible (actually several of them would simply become compatible based on existing sysnonyms) while not changing the actual tags on either of the two sites.

For the moment I tagged this , so that opinions on the genaral matter can be voiced and other suggestion to reseolve it might be made. Depending on how things progress I might change to .

Added: Thank you for the feedback. As a result of this dicsussion I placed a feature-request to remove this entirely with the request for synonyms to be taken into acount on meta.SO

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    $\begingroup$ Please do not discuss, here, whether the particular question should/should not be migrated, or the general merits of migration. $\endgroup$ – user9072 Dec 4 '13 at 16:45
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    $\begingroup$ I’d call this check “madness” rather than “sanity”. There is no sane reason why tags should be expected to be cross-site compatible (either in name or in their meaning), and for that matter, why should having no tags shared with the destination site prevent migration. Let the math.SE folks tag it as they see fit. $\endgroup$ – Emil Jeřábek Dec 4 '13 at 17:55
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    $\begingroup$ I'd go straight for asking this check to be removed entirely network-wide, I was pretty suprised that it even existed and I think this kind of check is fundamentally flawed and just not a good idea. $\endgroup$ – Mad Scientist Dec 4 '13 at 18:23
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    $\begingroup$ quid, if a question cannot be migrated, then perhaps two swallows could carry it on a string of creeper held under the dorsal guiding feathers, and carry it to Mercia. $\endgroup$ – Asaf Karagila Dec 4 '13 at 19:53
  • $\begingroup$ One way to solve this problem is that every time a migration fails for this reason, flag down Mariano and ask him to create the appropriate synonym at MSE. $\endgroup$ – Willie Wong Dec 11 '13 at 14:35
  • $\begingroup$ @WillieWong I do not really consider this as a solution but likely something like this is about the only wrokaround we have. Could you (or somebody else) perhaps follow through with meta.math.stackexchange.com/questions/12307/… at least for the most common ones; I think it will be more effecient to do this as a batch than literally doing what you suggest. $\endgroup$ – user9072 Jan 13 '14 at 12:07
  • $\begingroup$ @quid: my comment was supposed to be a joke.... but no matter. if I get some time I'll fill out some of the ones there. $\endgroup$ – Willie Wong Jan 16 '14 at 12:25
  • $\begingroup$ @WillieWong sorry about the confusion! I did not follow up on that comment than for reasoning along these lines, but then that meta thread reminded me of the issue. I think it would be really helpful if this could be done, for the topl-level tags at least, at least the large ones say number-theory, algebraic-geometry, combinatorics. $\endgroup$ – user9072 Jan 16 '14 at 13:03
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    $\begingroup$ @quid: like I said, slowly slowly I will put them in. Unfortunately the interface for entering synonyms rather deliberately slows down the process. $\endgroup$ – Willie Wong Jan 16 '14 at 13:08
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    $\begingroup$ @quid: I have entered most of them now. There are a few with some problems. I would appreciate comments on the answer that I posted on the link that you gave. $\endgroup$ – Willie Wong Jan 20 '14 at 11:27
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    $\begingroup$ We may also want to ask people at stats.stackexchange.com to add a couple of synonyms which will ease migration there. For example, a question at MathOverflow just failed to be migrated because it was tagged with 'st.statistics' only. $\endgroup$ – Ricardo Andrade Jan 20 '14 at 16:15
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    $\begingroup$ @RicardoAndrade good point! For stats it might suffice to get st.statistics to be an intrinisic tag there (most q should have it and if not it could be added by somebody). I strongly assume statistics is an intrinsic tag there already, but this does not help us as we have the prefix-version. (An intrinsic tag is a tag that is typically blacklisted on the site as too generic but in case of migration will be accepted as "valid" tag so the mig can happen though the tag is still removed).// Willie Wong: thanks! I commented there. $\endgroup$ – user9072 Jan 20 '14 at 22:06
  • $\begingroup$ @quid, any ideas on how to achieve such a thing at stats.stackexchange? Are there perhaps some MathOverflow users who can take care of it, like Willie Wong did at Math.StackExchange? $\endgroup$ – Ricardo Andrade Jan 20 '14 at 22:48
  • $\begingroup$ @RicardoAndrade I assume the actual creation needs to be done by developpers (not moderators). AFAIK there is a chat-room of all the different sites' moderators and developpers. I think this venue could be good for such a discussion (or something along these lines). That is, locally, we should get some moderators attention. Maybe via a new meta question. $\endgroup$ – user9072 Jan 21 '14 at 10:54
  • $\begingroup$ @RicardoAndrade another idea could be to ask on the stats site to make st.statistics a synonym of mathematical-statistics a tag they have. Perhaps one (you or I) simply should go ahaed and ask on their meta regarding this. $\endgroup$ – user9072 Jan 22 '14 at 19:41

The issue is every question must have a tag. Those tags that do not exist on the target site get removed during migration. So if none of the tags exist on the target site it has no tags and the engine would reject the question.

If I am not mistaken it used to be the case that in such cases the migrated question would get tagged with so the questions would not get rejected in such cases. I am guessing the current situation is the unintended side effect of some other change.

  • $\begingroup$ Yes I was aware of this issue (and the fix via using untagged also came to mind). And, I do not think this is an unintended sideeffect of something. As visible in the linked to post Tim Post explained this was like this and it is documented in the FAQ on migration on meta.SO $\endgroup$ – user9072 Dec 5 '13 at 12:47
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    $\begingroup$ @quid, I see. Still I am not sure if it was an intentional decision. I am pretty sure this is new and it wasn't this way before as I have migrated a number of questions to Computer Science from Theoretical Computer Science without any valid tags on Computer Science. One solution is to create tag synonyms on the target site corresponding to those on the original site, we did this for a few more popular tags so we don't need to manually fix the tags after the migration. $\endgroup$ – Kaveh Dec 5 '13 at 14:23
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    $\begingroup$ That is an intentional side effect, I asked Tim Post about it when this came up (see this chat message and a few after that (mod-only)) $\endgroup$ – Mad Scientist Dec 5 '13 at 19:54
  • $\begingroup$ @MadScientist, I see. $\endgroup$ – Kaveh Dec 5 '13 at 20:52
  • $\begingroup$ Your solution is somehow dual to what I suggest; personally, I consider it as desirable if this can be (also) done on the source site (and IMO there is no reason at all for not checking for those, basically this seems like an oversight/bug to me). In addition, for the specific case of MO and math.SE it would be especially helpful if the synonyms on the source site would also be taken into account as a lot of the problem in practice here is due to our top-level prefixes and we have anyway synonyms for the nonprefixed tags, whil to create the prefixed one on math.SE seems weird. $\endgroup$ – user9072 Dec 5 '13 at 22:40

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