How to query an optional column on a self referential adjacency list table?

[Jinghui Niu]

I have the following Table model representing a timeline.

class TimeRange(Base):


    __tablename__ = "time_line"


    record_id = Column(Integer, primary_key=True)
    level = Column(String, nullable=False) # e.g. "Point", "Range"
    content = Column(String, nullable=False)
    language_marker = Column(String) # this one column is optional and needs to be queried
    immediate_parent_id = Column(Integer, ForeignKey('time_line.record_id'))
    child_timelines = relationship('TimeRange', backref=backref('parent_timeline', remote_side=[record_id]))

The language_marker Column is the one that needs to be queried in a recursive manner. Not all records have such an attribute, and the business logic is: along the hierarchy lineage from the root down to the child timelines, at least one level of the TimeRange instance carries such an attribute, and the one in the lowest level should be returned. This works a little like cascading style sheet, where if the TimeRange object itself doesn't have such an attribute, just look further up one level above, util found one, and the latest defined style wins.

What is the technical direction I should look into to implement such queries? I'm using SQLAlchemy and the backend is SQLite. Thanks.

[Jinghui Niu]

I'm thinking of JOIN with the parents, but failed. Because joining indefinite levels seems to be very hard to implement. You don't know how many levels you need to look up until you find a parent with a non Null value for that optional column. How to approach this problem?

[Mike Bayer]

On 10/14/2016 06:08 PM, Jinghui Niu wrote:
> I have the following Table model representing a timeline.
>
> |
> classTimeRange(Base):
>
>
> __tablename__ ="time_line"
>
>
> record_id =Column(Integer,primary_key=True)

> level =Column(String,nullable=False)# e.g. "Point", "Range"
> content =Column(String,nullable=False)
> language_marker =Column(String)# this one column is optional and

> needs to be queried
> immediate_parent_id =Column(Integer,ForeignKey('time_line.record_id'))
> child_timelines

> =relationship('TimeRange',backref=backref('parent_timeline',remote_side=[record_id]))

> |
>
>
> The language_marker Column is the one that needs to be queried in a
> recursive manner. Not all records have such an attribute, and the
> business logic is: along the hierarchy lineage from the root down to the
> child timelines, at least one level of the TimeRange instance carries
> such an attribute, and the one in the lowest level should be returned.
> This works a little like cascading style sheet, where if the TimeRange
> object itself doesn't have such an attribute, just look further up one
> level above, util found one, and the latest defined style wins.
>
> What is the technical direction I should look into to implement such
> queries? I'm using SQLAlchemy and the backend is SQLite. Thanks.

Someone else can probably work out the details on this one, but the
general technique on the SQL side is to use a recursive query. With
Postgresql / SQL Server this is a CTE using WITH RECURSIVE, with Oracle
I *think* they support this syntax also now though historically it's
been "CONNECT BY", and then with any other DB like MySQL / SQlite it's
basically nothing.

The other technique, which I tend to prefer if it can be made feasible,
is that if I'm working with overall a limited number of rows in the
first place, such as all of these records where language_marker may be
significant all belong to some common "document id" or something where
there are only a few hundred or a few thousand rows that would matter
for the whole operation I'm doing, I pull it into memory and assemble it
into a tree hierarchy right there.


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[Jinghui Niu]

Thanks Mike.

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[Mike Bayer]

oh I was wrong, SQLite has it:

https://www.sqlite.org/lang_with.html


that leaves just MySQL out.

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[Jinghui Niu]

Do I need to turn on or do some configuration with SQLAlchemy before using CTE? Like foreign keys

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