[postgis-users] Saving a split polygon

Sat Oct 12 10:39:33 PDT 2013

so, what was the answer?

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On Sat, Oct 12, 2013 at 1:37 PM, Peter Brown <peterbrown.kew at gmail.com>wrote:

> Dear Alexandre
> Sorry I have not replied before, but your suggestions have set me off on a
> new voyage of discovery. It has been exceeding helpful, so thank you very
> much
> Regards
> Peter
>
>
> On 7 October 2013 16:33, Alexandre Neto <senhor.neto at gmail.com> wrote:
>
>> Hello Peter,
>>
>> I'm not a database expert whatsoever, and there are better people in this
>> list to clarify this. I just stated my experience with it.
>>
>> As far as I know, in order to respect Postgres Primary keys, QGIS uses
>> the columns default values.
>>
>> If during the table creation your primary keys were created with
>> something like:
>>
>> CREATE TABLE yourtable
>>  (
>> gid serial NOT NULL,
>> ...
>> )
>>
>> Then, a sequence 'yourtable_gid_seq' is created and the default value
>> for your gid column is set to nextval('yourtable_gid_seq'::regclass)
>>
>> When a new feature(row) is created if the user does not fill the pk field
>> (and he is not supposed to), postgres will consider the default value, give
>> it the next not use value from the sequence and update the sequence.
>>
>> If you are using QGIS, when you create a new feature or simply split an
>> existing features, make sure not to type anything in the PK field, or this
>> will give you a bad time. Just leave it blank, during the save\commit, both
>> QGIS and Postgres will know what do do with it.
>>
>> Not sure where all this knowledge came from, but have a look here:
>> http://www.postgresql.org/docs/9.1/static/functions-sequence.html
>>
>> Best regards,
>> Alexandre Neto
>>
>>
>> On Mon, Oct 7, 2013 at 3:59 PM, Peter Brown <peterbrown.kew at gmail.com>wrote:
>>
>>> uummmm
>>> Reading your answer {particularly the bit about  a
>>> nextval('yourtable_gid_seq'::regclass),} I realise that this bit of
>>> code is an area of knowledge that I am missing, and clearly if it will
>>> solve my problem then it would be useful. I wonder if I could ask you to at
>>> least point me in the right direction.
>>> I am fairly new to QGIS but generally a power user of databases, so I
>>> have read the excellent manual
>>> http://workshops.opengeo.org/postgis-intro/creating_db.html
>>> and the more fundamental one at
>>> http://www.postgresql.org/docs/9.2/interactive/sql.html
>>> However, I do not seem to have come across anything like this. Can you
>>> advise me?
>>> Thank you for your help
>>> Regards
>>> Peter Brown
>>>
>>>
>>> On 7 October 2013 11:09, Alexandre Neto <senhor.neto at gmail.com> wrote:
>>>
>>>> Hello,
>>>>
>>>> I'm not sure If I followed your question, but in my experience with
>>>> QGIS editing Postgres\postgis spatial tables, I add some smal hickups, but
>>>> never with splitting.
>>>>
>>>> If its a normal polygon and you wish to split it with a line, QGIS will
>>>> assign to all your new polygons a nextval('yourtable_gid_seq'::regclass),
>>>> and this should solve the PK's problem.
>>>>
>>>> If it's a multipart polygon that you want to spit it's parts, you can
>>>> use the multipartsplit plugin, and it will also take care of your primary
>>>> keys.
>>>>
>>>> Maybe it's a problem with your table, please send the creation SQL
>>>> command and maybe someone can have a clue of what is happening.
>>>>
>>>>
>>>>
>>>> On Mon, Oct 7, 2013 at 10:41 AM, Peter Brown <peterbrown.kew at gmail.com>wrote:
>>>>
>>>>> Thanks for your prompt response, it is nice to know that it is
>>>>> possible, but the methods you remember do seem to have problems.
>>>>> - in QGIS I can split a polygon which is on a layer, and under the
>>>>> 'layer' option on the menu, I call up the attributes table, but that only
>>>>> offers me all the fields in the base PostGIS table EXCEPT   (a) the  ID and
>>>>> (b) the geometry
>>>>> - I have tried manufacturing another PostGIS table with all the
>>>>> attributes of the polygon except an ID field with all its 'NO NULLS' etc,
>>>>> but then when I open a layer in QGIS it draws the polygon but shows it as
>>>>> not editable
>>>>> You have obviously managed something clever, but I am not quite sure
>>>>> what
>>>>> Regards
>>>>> Peter Brown
>>>>>
>>>>>
>>>>> On 7 October 2013 09:32, Hugues François <hugues.francois at irstea.fr>wrote:
>>>>>
>>>>>> **
>>>>>> Hello,
>>>>>>
>>>>>> I previously edited polygons from postgis within qgis and with no
>>>>>> problem. If I remember well, before saving I also filled attribute data to
>>>>>> give a new ID. Maybe if you just want to edit polygons without changing
>>>>>> attributes, you may remove constraints on your postgis table (especially,
>>>>>> uniques, not null and primary key) before editing (and then put the things
>>>>>> right back to postgis).
>>>>>>
>>>>>> Hugues.
>>>>>>
>>>>>>
>>>>>>
>>>>>>  ------------------------------
>>>>>> *From:* postgis-users-bounces at lists.osgeo.org [mailto:
>>>>>> postgis-users-bounces at lists.osgeo.org] *On Behalf Of *Peter Brown
>>>>>> *Sent:* Monday, October 07, 2013 9:53 AM
>>>>>> *To:* postgis-users at lists.osgeo.org
>>>>>> *Subject:* [postgis-users] Saving a split polygon
>>>>>>
>>>>>>  In Quantum GIS (QGIS) there is a very useful facility for taking a
>>>>>> polygon in a PostGIS database linked to QGIS and splitting it. HOWEVER,
>>>>>> when one then tries to save the two portions, PostGIS puts up an error
>>>>>> message that it cannot save the new portion because it does not have a new
>>>>>> ID (it can save the existing portion, because this is only an edit of an
>>>>>> existing ID).
>>>>>> There are workarounds for this (e.g. convert to 'Shape' files) but
>>>>>> they seem incredibly clumsy. Does anyone have a more direct solution?
>>>>>> Thanks
>>>>>> Peter Brown
>>>>>>
>>>>>>
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>>>>>
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