[postgis-users] Saving a split polygon

[Peter Brown]

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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