Hi,
I'm running foreman-2.0.0-2.el7.noarch with PostGreSQL 11. In the web-ui I adjusted the filtered facts list and wanted to cleanup my database:

# foreman-rake facts:clean
Starting orphaned facts clean up
rake aborted!
ActiveRecord::InvalidForeignKey: PG::ForeignKeyViolation: ERROR:  update or delete on table "fact_names" violates foreign key constraint "fact_values_fact_name_id_fk" on table "fact_values" 
DETAIL:  Key (id)=(202932) is still referenced from table "fact_values".
: DELETE FROM "fact_names" WHERE "fact_names"."id" IN ($1, $2, $3, $4, $5, $6, $7, $8, $9, $10, $11, $12, $13, $14, $15, $16, $17, $18, $19, $20, $21, $22, $23, $24, $25, $26, $27, $28, $29, $30, $31, $32, $33, $34, $35, $36, $37, $38, $39, $40, $41, $42, $43, $44, $45, $46, $47, $48, $49, $50, $51, $52, $53, $54, $55, $56, $57, $58, $59, $60, $61, $62, $63, $64, $65, $66, $67, $68, $69, $70, $71, $72, $73, $74, $75, $76, $77, $78, $79, $80, $81, $82, $83, $84, $85, $86, $87, $88, $89, $90, $91, $92, $93, $94, $95, $96, $97, $98, $99, $100, $101, $102, $103, $104, $105, $106, $107, $108, $109, $110, $111, $112, $113, $114, $115, $116, $117, $118, $119, $120, $121, $122, $123, $124, $125, $126, $127, $128, $129, $130, $131, $132, $133, $134, $135, $136, $137, $138, $139, $140, $141, $142, $143, $144, $145, $146, $147, $148, $149, $150, $151, $152, $153, $154, $155, $156, $157, $158, $159, $160, $161, $162, $163, $164, $165, $166, $167, $168, $169, $170, $171, $172, $173, $174, $175, $176, $177, $178, $179, $180, $181, $182, $183, $184, $185, $186, $187, $188, $189, $190, $191, $192, $193, $194, $195, $196, $197, $198, $199, $200, $201, $202, $203, $204, $205, $206, $207, $208, $209, $210, $211, $212, $213, $214, $215, $216, $217, $218, $219, $220, $221, $222, $223, $224, $225, $226, $227, $228, $229, $230, $231, $232, $233, $234, $235, $236, $237, $238, $239, $240, $241, $242, $243, $244, $245, $246, $247, $248, $249, $250, $251, $252, $253, $254, $255, $256, $257, $258, $259, $260, $261, $262, $263, $264, $265, $266, $267, $268, $269, $270, $271, $272, $273, $274, $275, $276, $277, $278, $279, $280, $281, $282, $283, $284, $285, $286, $287, $288, $289, $290, $291, $292, $293, $294, $295, $296, $297, $298, $299, $300, $301, $302, $303, $304, $305, $306, $307, $308, $309, $310, $311, $312, $313, $314, $315, $316, $317, $318, $319, $320, $321, $322, $323, $324, $325, $326, $327, $328, $329, $330, $331, $332, $333, $334, $335, $336, $337, $338, $339, $340, $341, $342, $343, $344, $345, $346, $347, $348, $349, $350, $351, $352, $353, $354, $355, $356, $357, $358, $359, $360, $361, $362, $363, $364, $365, $366, $367, $368, $369, $370, $371, $372, $373, $374, $375, $376, $377, $378, $379, $380, $381, $382, $383, $384, $385, $386, $387, $388, $389, $390, $391, $392, $393, $394, $395, $396, $397, $398, $399, $400, $401, $402, $403, $404, $405, $406, $407, $408, $409, $410, $411, $412, $413, $414, $415, $416, $417, $418, $419, $420, $421, $422, $423, $424, $425, $426, $427, $428, $429, $430, $431, $432, $433, $434, $435, $436, $437, $438, $439, $440, $441, $442, $443, $444, $445, $446, $447, $448, $449, $450, $451, $452, $453, $454, $455, $456, $457, $458, $459, $460, $461, $462, $463, $464, $465, $466, $467, $468, $469, $470, $471, $472, $473, $474, $475, $476, $477, $478, $479, $480, $481, $482, $483, $484, $485, $486, $487, $488, $489, $490, $491, $492, $493, $494, $495, $496, $497, $498, $499, $500)
/opt/theforeman/tfm/root/usr/share/gems/gems/activerecord-5.2.1/lib/active_record/connection_adapters/postgresql_adapter.rb:603:in `async_exec'
/opt/theforeman/tfm/root/usr/share/gems/gems/activerecord-5.2.1/lib/active_record/connection_adapters/postgresql_adapter.rb:603:in `block (2 levels) in exec_no_cache'
/opt/theforeman/tfm/root/usr/share/gems/gems/activesupport-5.2.1/lib/active_support/dependencies/interlock.rb:48:in `block in permit_concurrent_loads'
/opt/theforeman/tfm/root/usr/share/gems/gems/activesupport-5.2.1/lib/active_support/concurrency/share_lock.rb:187:in `yield_shares'
/opt/theforeman/tfm/root/usr/share/gems/gems/activesupport-5.2.1/lib/active_support/dependencies/interlock.rb:47:in `permit_concurrent_loads'
/opt/theforeman/tfm/root/usr/share/gems/gems/activerecord-5.2.1/lib/active_record/connection_adapters/postgresql_adapter.rb:602:in `block in exec_no_cache'
/opt/theforeman/tfm/root/usr/share/gems/gems/activerecord-5.2.1/lib/active_record/connection_adapters/abstract_adapter.rb:579:in `block (2 levels) in log'
/opt/theforeman/tfm/root/usr/share/gems/gems/activerecord-5.2.1/lib/active_record/connection_adapters/abstract_adapter.rb:578:in `block in log'
/opt/theforeman/tfm/root/usr/share/gems/gems/activesupport-5.2.1/lib/active_support/notifications/instrumenter.rb:23:in `instrument'
/opt/theforeman/tfm/root/usr/share/gems/gems/activerecord-5.2.1/lib/active_record/connection_adapters/abstract_adapter.rb:569:in `log'
/opt/theforeman/tfm/root/usr/share/gems/gems/activerecord-5.2.1/lib/active_record/connection_adapters/postgresql_adapter.rb:601:in `exec_no_cache'
/opt/theforeman/tfm/root/usr/share/gems/gems/activerecord-5.2.1/lib/active_record/connection_adapters/postgresql_adapter.rb:590:in `execute_and_clear'
/opt/theforeman/tfm/root/usr/share/gems/gems/activerecord-5.2.1/lib/active_record/connection_adapters/postgresql/database_statements.rb:94:in `exec_delete'
/opt/theforeman/tfm/root/usr/share/gems/gems/activerecord-5.2.1/lib/active_record/connection_adapters/abstract/database_statements.rb:163:in `delete'
/opt/theforeman/tfm/root/usr/share/gems/gems/activerecord-5.2.1/lib/active_record/connection_adapters/abstract/query_cache.rb:21:in `delete'
/opt/theforeman/tfm/root/usr/share/gems/gems/activerecord-5.2.1/lib/active_record/relation.rb:401:in `delete_all'
/usr/share/foreman/app/services/fact_cleaner.rb:47:in `block in delete_orphaned_facts'
/opt/theforeman/tfm/root/usr/share/gems/gems/activerecord-5.2.1/lib/active_record/relation/batches.rb:136:in `block in find_in_batches'
/opt/theforeman/tfm/root/usr/share/gems/gems/activerecord-5.2.1/lib/active_record/relation/batches.rb:238:in `block in in_batches'
/opt/theforeman/tfm/root/usr/share/gems/gems/activerecord-5.2.1/lib/active_record/relation/batches.rb:222:in `loop'
/opt/theforeman/tfm/root/usr/share/gems/gems/activerecord-5.2.1/lib/active_record/relation/batches.rb:222:in `in_batches'
/opt/theforeman/tfm/root/usr/share/gems/gems/activerecord-5.2.1/lib/active_record/relation/batches.rb:135:in `find_in_batches'
/usr/share/foreman/app/services/fact_cleaner.rb:45:in `delete_orphaned_facts'
/usr/share/foreman/app/services/fact_cleaner.rb:11:in `clean!'
/usr/share/foreman/lib/tasks/facts.rake:15:in `block (2 levels) in <top (required)>'
/opt/rh/rh-ruby25/root/usr/share/gems/gems/rake-12.3.0/exe/rake:27:in `<top (required)>'

Caused by:
PG::ForeignKeyViolation: ERROR:  update or delete on table "fact_names" violates foreign key constraint "fact_values_fact_name_id_fk" on table "fact_values" 
DETAIL:  Key (id)=(202932) is still referenced from table "fact_values".
/opt/theforeman/tfm/root/usr/share/gems/gems/activerecord-5.2.1/lib/active_record/connection_adapters/postgresql_adapter.rb:603:in `async_exec'
/opt/theforeman/tfm/root/usr/share/gems/gems/activerecord-5.2.1/lib/active_record/connection_adapters/postgresql_adapter.rb:603:in `block (2 levels) in exec_no_cache'
/opt/theforeman/tfm/root/usr/share/gems/gems/activesupport-5.2.1/lib/active_support/dependencies/interlock.rb:48:in `block in permit_concurrent_loads'
/opt/theforeman/tfm/root/usr/share/gems/gems/activesupport-5.2.1/lib/active_support/concurrency/share_lock.rb:187:in `yield_shares'
/opt/theforeman/tfm/root/usr/share/gems/gems/activesupport-5.2.1/lib/active_support/dependencies/interlock.rb:47:in `permit_concurrent_loads'
/opt/theforeman/tfm/root/usr/share/gems/gems/activerecord-5.2.1/lib/active_record/connection_adapters/postgresql_adapter.rb:602:in `block in exec_no_cache'
/opt/theforeman/tfm/root/usr/share/gems/gems/activerecord-5.2.1/lib/active_record/connection_adapters/abstract_adapter.rb:579:in `block (2 levels) in log'
/opt/theforeman/tfm/root/usr/share/gems/gems/activerecord-5.2.1/lib/active_record/connection_adapters/abstract_adapter.rb:578:in `block in log'
/opt/theforeman/tfm/root/usr/share/gems/gems/activesupport-5.2.1/lib/active_support/notifications/instrumenter.rb:23:in `instrument'
/opt/theforeman/tfm/root/usr/share/gems/gems/activerecord-5.2.1/lib/active_record/connection_adapters/abstract_adapter.rb:569:in `log'
/opt/theforeman/tfm/root/usr/share/gems/gems/activerecord-5.2.1/lib/active_record/connection_adapters/postgresql_adapter.rb:601:in `exec_no_cache'
/opt/theforeman/tfm/root/usr/share/gems/gems/activerecord-5.2.1/lib/active_record/connection_adapters/postgresql_adapter.rb:590:in `execute_and_clear'
/opt/theforeman/tfm/root/usr/share/gems/gems/activerecord-5.2.1/lib/active_record/connection_adapters/postgresql/database_statements.rb:94:in `exec_delete'
/opt/theforeman/tfm/root/usr/share/gems/gems/activerecord-5.2.1/lib/active_record/connection_adapters/abstract/database_statements.rb:163:in `delete'
/opt/theforeman/tfm/root/usr/share/gems/gems/activerecord-5.2.1/lib/active_record/connection_adapters/abstract/query_cache.rb:21:in `delete'
/opt/theforeman/tfm/root/usr/share/gems/gems/activerecord-5.2.1/lib/active_record/relation.rb:401:in `delete_all'
/usr/share/foreman/app/services/fact_cleaner.rb:47:in `block in delete_orphaned_facts'
/opt/theforeman/tfm/root/usr/share/gems/gems/activerecord-5.2.1/lib/active_record/relation/batches.rb:136:in `block in find_in_batches'
/opt/theforeman/tfm/root/usr/share/gems/gems/activerecord-5.2.1/lib/active_record/relation/batches.rb:238:in `block in in_batches'
/opt/theforeman/tfm/root/usr/share/gems/gems/activerecord-5.2.1/lib/active_record/relation/batches.rb:222:in `loop'
/opt/theforeman/tfm/root/usr/share/gems/gems/activerecord-5.2.1/lib/active_record/relation/batches.rb:222:in `in_batches'
/opt/theforeman/tfm/root/usr/share/gems/gems/activerecord-5.2.1/lib/active_record/relation/batches.rb:135:in `find_in_batches'
/usr/share/foreman/app/services/fact_cleaner.rb:45:in `delete_orphaned_facts'
/usr/share/foreman/app/services/fact_cleaner.rb:11:in `clean!'
/usr/share/foreman/lib/tasks/facts.rake:15:in `block (2 levels) in <top (required)>'
/opt/rh/rh-ruby25/root/usr/share/gems/gems/rake-12.3.0/exe/rake:27:in `<top (required)>'
Tasks: TOP => facts:clean
(See full trace by running task with --trace)

I searched for the ID in the database:

foreman=# SELECT COUNT(id) FROM fact_values WHERE fact_name_id = 202932;
 count 
-------
    24
(1 row)

foreman=# SELECT * FROM fact_names WHERE id = 202932;
   id   |                           name                            |         updated_at         |         created_at         | compose | short_name |      type      |           ancestry           
--------+-----------------------------------------------------------+----------------------------+----------------------------+---------+------------+----------------+------------------------------
 202932 | lldp::interface::eth0::port::auto-negotiation::advertised | 2017-03-20 08:15:30.996328 | 2017-03-20 08:15:30.996328 | t       | advertised | PuppetFactName | 878/879/202921/202930/202931
(1 row)

please let me know if you need additional information. I'm also available on IRC as bastelfreak.